Forecast Discussions mentioning any of
"HRRR" "RAP" "RUC13" "RUC" "RR" received at GSD on 07/22/20
Area Forecast Discussion
National Weather Service Cleveland OH
920 PM EDT Tue Jul 21 2020
High pressure will move east of the area tonight as a warm front
lifts northeast across the area. Low pressure will move into the
western Great Lakes late tonight. As the low pressure system
moves east toward Maine, a pre-frontal trough will move east
during the afternoon tomorrow. A cold front trailing the trough
will move east across the area tomorrow night. Another weak low
pressure system will move southeast across the area Thursday
night followed by a high pressure ridge that will build
southeast over the area through the weekend.
.NEAR TERM /THROUGH WEDNESDAY NIGHT/...
Previously mentioned convective activity that skirted the
southern part of the forecast area has now moved southeast
towards the Ohio River. Focus turns to upstream convective
activity across central IN, with a well developed MCS moving
into the Indianapolis metro within the next hour. While models
generally are not capturing the scope of this activity, the HRRR
seems to be initializing it somewhat, although at a diminished
and delayed capacity. Generally expect the bulk of the activity
to weaken as it moves into Ohio, with the stronger convection
generally staying south of the forecast area. Still expecting
some weaker convection/precip to move into the Findlay area
sometime in the 05-06Z time frame as the low level jet noses
northeastward towards the area, with 500-1000 j/kg MUCAPE
residing along the I-75 corridor.
A fast moving upper level shortwave trough will move east
across the Great Lakes region during this period. This upper
level feature will trigger a weak wave of low pressure at the
surface over Minnesota that will deepen tonight through
Wednesday as it moves into the Great Lakes region.
A line of convection moving east across Indiana at this time is
expected to weaken before it arrives in our area tonight. However,
as the strong positive vorticity maximum and shortwave trough arrive
over the area late tonight, expecting another round of showers and
thunderstorms to move east over the area tonight into tomorrow
morning. This activity was currently over Missouri.
The surface low pressure system will force a warm front northeast
across the area tonight followed by a pre-frontal trough tomorrow
afternoon and a cold front tomorrow night. As the surface features
move through the area, moisture associated with the system will be
present and support threat for rain across the area. Latest
satellite data shows precipitable water values around 1.9" advecting
north with the warm front. This will support the potential for a
very moist environment tomorrow when periods of moderate to heavy
rain will occur in the convection.
During the overnight period, forecast area is outlooked for a
marginal risk for severe thunderstorms over the western quarter of
the forecast area. General thunder is expected elsewhere. The day
2 outlook suggests a marginal risk for severe thunderstorms over the
entire forecast area for tomorrow.
Warm air advection will begin to return tonight in the wake of
the warm front. This will allow temperatures to stay in the
lower to middle 60s extreme east to upper 60s and lower 70s
elsewhere. Highs tomorrow will be in the middle to upper 80s
over much of the area with the exception of northwest
Pennsylvania where highs will be in the lower 80s. Tomorrow
night`s lows will drop slightly into the middle to upper 60s due
to passage of the cold front.
.SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/...
A cold front is expected to move southeast across the area early
Thursday morning with some scattered showers/thunderstorms possible
in the afternoon/evening hours. Temperatures will be right around
average behind the cold front. High pressure begins building in from
the northwest on Friday, though an isolated showers/thunderstorm
could still be possible Friday afternoon with weak instability
developing with diurnal heating. Temperatures on Friday will be very
similar to Thursday.
.LONG TERM /SATURDAY THROUGH TUESDAY/...
High pressure expected to have a stronger influence on the local
weather on Saturday with mostly sunny skies and no precipitation
chances. Hot weather returns on Sunday and Monday as high pressure
departs to our east, with high temperatures in the low 90s expected.
With high dew points in the low 70s possible, heat indices in the
mid to upper 90s may be possible on Sunday/Monday. Slightly cooler
temperatures expected on Tuesday as models key in on an approaching
upper-level trough. Isolated to scattered thunderstorms expected
Sunday through Tuesday, mainly during the afternoon hours with
.AVIATION /00Z Wednesday THROUGH Sunday/...
Generally lower confidence TAF forecast over the next 12-15
hours. Ongoing decaying SHRA/TSRA will briefly impact KMFD, and
may impact KCAK over the next hour or two. Otherwise, focus of
the forecast is SHRA/TSRA potential generally after 06Z from
west to east across the area. Highest confidence for SHRA/TSRA
and ceiling/vis reductions is at KFDY, with lowering confidence
farther east through the 12Z-15Z window. A break in precip and
generally VFR conditions expected mid morning through the
afternoon, with more TSRA potential late in the period as a cold
front moves southeast across the region. Light/variable winds
through much of the overnight will increase a bit our of the
west/southwest by midday Wednesday.
Outlook...Occasional non-VFR possible through Wednesday night
and again possible Sunday afternoon.
A warm front is expected to lift north across Lake Erie late this
evening into early tonight, allowing southerly winds of 10 knots or
less to develop during the overnight hours tonight. Southwest winds
of 15 knots are then expected during the daytime hours Wednesday. A
cold front will then move southeast across Lake Erie late Wednesday
night into early Thursday morning allowing west to northwest winds
of 10 knots to develop on Thursday. High pressure builds east across
the Great Lakes region on Friday and Saturday with relatively weak
winds on both of those days. By Saturday night, a warm front lifts
north across Lake Erie with southwest winds of 10 to 15 knots
developing on Lake Erie Sunday and Monday.
Area Forecast Discussion
National Weather Service Cheyenne WY
527 PM MDT Tue Jul 21 2020
.SHORT TERM...(Late this afternoon through Thursday)
Issued at 315 PM MDT Tue Jul 21 2020
Latest VIS satellite imagery showed towering CU along the Laramie
Range, with isolated thunderstorms indicated by KCYS radar. Convection
is forming along the MUCAPE axis of 2000-2500 j/kg per SPC mesoanalysis
and stalled surface boundary. Anticipate convection increasing in
coverage and intensity through late afternoon, with storm motion to
the ESE. Could be a few right-moving supercells per model hodographs.
19Z HRRR continues convective trends with scattered strong to severe
convection affecting the I-25 corridor and high plains of southeast
WY and much of the western NE Panhandle through mid-evening. 0-6km
shear is 25-35 kt, with primary hazards large hail up to 2 inches and
damaging wind gusts of 60 to 70 mph. Convection will diminish late
this evening across western NE.
Upper ridge will be centered over the south-central Plains Wednesday
and Thursday. Surface boundary will begin a slow motion to the east,
with drier air aloft spreading to the east into the high plains.
Anticyclonic circulation around the upper high will advect
subtropical/monsoonal moisture into the CWA. Scattered convection
will develop during the afternoon and linger into the evening. Best
instability and shear will be located east of the CWA, with a limited
severe threat for the southeast NE Panhandle. A similar convective
pattern is expected Thursday with a limited severe threat for the
northeast NE Panhandle. Above normal temperatures will prevail with
daytime highs in the 80s and 90s and nighttime lows in the 50s and
.LONG TERM...(Thursday Night through Monday)
Issued at 406 AM MDT Tue Jul 21 2020
Upper level ridge axis begins to shift slightly to the east over the
central CONUS by Thursday evening. On the heels of the departing
ridge axis from the CWA is a shortwave trough that will slowly
propagate south and east through Friday. The associated cold front
looks to stay north of the area however, thus daytime highs should
still be very warm for this time of year for Friday. As the
shortwave trough stays further to the north of our forecast region
as it propagates eastward for Friday into the weekend, the monsoon
moisture will begin to slowly make its way northward over the
intermountain west. Medium range guidance for Friday through the
weekend show a subtropical high advecting the moisture northward
thanks to the clockwise airflow over Mexico.
Model disagreements are still present however on the timing of the
moisture advecting into the area. The Euro and Canadian model
guidance show several impulses of moisture by Sunday and Sunday
night making its way over the CWA, but the GFS has a slower solution
that is also noted to be slightly eastward of the CWA leaving the
region with less moisture overall. Weak upper level ridging to
potential zonal flow for Tuesday shows the area warming back up
slightly. If the long term models agree on a solution with the upper
level ridging, we could be looking at near record highs to close out
the end of the month of July.
.AVIATION...(For the 00Z TAFS through 00Z Wednesday evening)
Issued at 521 PM MDT Tue Jul 21 2020
Wyoming TAFS...VFR, with occasional thunderstorms producing MVFR
and wind gusts to 45 knots at Laramie and Cheyenne until 02Z. Wind
gusts to 23 knots at all terminals after 15Z Wednesday.
Nebraska TAFS...VFR at Chadron and Alliance, with occasional
thunderstorms producing MVFR and wind gusts to 35 knots from 01Z
to 03Z. Wind gusts to 27 knots after 15Z Wednesday.
VFR at Scottsbluff until 06Z, with occasional thunderstorms
producing MVFR and wind gusts to 45 knots from 01Z to 03Z, then
MVFR from 06Z to 15Z, then VFR. Wind gusts to 24 knots after 15Z
VFR at Sidney, with occasional thunderstorms producing MVFR and
wind gusts to 45 knots from 01Z to 03Z. Wind gusts to 25 knots
after 15Z Wednesday.
Issued at 225 PM MDT Tue Jul 21 2020
Scattered thunderstorms will develop along the Front Range this
afternoon and move east onto the plains through this evening. A
stalled frontal boundary will shift slowly east of the Front Range
Wednesday, and into the plains Thursday. Drier air west of the
Laramie Range will spread into southeast Wyoming and western
Nebraska later this week. Isolated to scattered afternoon and
evening thunderstorms will continue this weekend and early next
week. Winds will be light, except near thunderstorms.
Area Forecast Discussion
National Weather Service Goodland KS
908 PM MDT Tue Jul 21 2020
Issued at 907 PM MDT Tue Jul 21 2020
Confidence in storm activity lasting into Nebraska is on the low
side based on current data. Therefore have lowered rainfall
chances for the night.
.SHORT TERM...(This evening through Wednesday night)
Issued at 137 PM MDT Tue Jul 21 2020
Sunny skies prevailed across the region early this afternoon, with
a weak frontal boundary stalled along the Nebraska border. Satellite
and 500mb RAP analysis showed northwest flow aloft as a shortwave
trough moved into eastern Kansas and high pressure built over the
desert southwest. At 1:30 PM MT, temperatures ranged in the 80s with
east/southeast winds at 10 to 15 mph.
Tonight...Chances for showers and thunderstorms return to the
forecast late this afternoon through the overnight hours. A few
storms are anticipated to form along the Front Range and push east
into eastern Colorado late this afternoon where diurnal heating will
aid the development of roughly 1000-1500 J/kg of MLCAPE. Coverage
should be isolated and at this time, it is very hard to say how far
east storms will make it due to weak capping that will be in place.
Nevertheless, a few strong to severe storms will be possible, mainly
west of Highway 25, from the late afternoon into the evening. Large
hail and damaging winds will be the main threats. Additionally,
locally heavy rainfall could cause flash flooding.
Guidance does suggest that an area of convection meanders along the
Nebraska border through the overnight hours/early Wednesday. With
these storms moving into an unstable environment, cannot rule out
isolated instances of large hail and strong winds. Otherwise, expect
lows in the 60s.
Wednesday and Wednesday night...Storms may linger across the
northeastern portion of the region early Wednesday morning before
exiting around sunrise. Sunny skies are expected midday with high
temperatures in the 90s. Southerly winds will be breezy, gusting up
to 30 mph in the afternoon and evening.
Another round of thunderstorms is forecast to form along a lee
trough in the afternoon and evening. Although uncertainty remains on
exactly where storms will develop, the best chance for severe
weather will be along and north of Interstate 70, with large hail,
damaging winds, and heavy rainfall possible. Low temperatures fall
into the 60s/low 70s overnight.
.LONG TERM...(Thursday through Tuesday)
Issued at 118 PM MDT Tue Jul 21 2020
At the start of the extended period the models are showing the
local area under the influence of a strong upper-level high and
ridge over the Plains that is producing weal southwesterly flow
aloft over the Central High Plains region. This pattern looks to
be the dominating feature for the period. Above normal
temperatures (middle 90s) are anticipated through the weekend
before we see a slight cooling (upper 80 to lower 90s) on Monday.
Daily chances for showers and thunderstorms are possible during
the extended forecast, mainly in the afternoon and evening hours.
However, confidence in widespread moisture is low at this time.
.AVIATION...(For the 00Z TAFS through 00Z Wednesday evening)
Issued at 546 PM MDT Tue Jul 21 2020
VFR conditions forecast for the TAFs. Main focus is whether any
thunderstorms will move near KMCK around 6z tonight. At this time
confidence is low enough to leave out a mention in the TAF.
Current data suggests the storm coverage will either remain north
of the site or dwindle in coverage as it approaches the site.
Area Forecast Discussion
National Weather Service Hanford CA
131 PM PDT Tue Jul 21 2020
A couple afternoon thunderstorms are again possible over the
higher elevations of the Sierra Nevada this afternoon. Slightly
cooler tomorrow and Thursday as low pressure moves through, with a
rebound back to normal temperatures this weekend.
The low pressure system that is nearing the California coastline
is bringing some high level cloud cover today along with slightly
lower afternoon highs. Some cumulus clouds are forming over the
Sierra Nevada as of this writing and may evolve into isolated
thunderstorms as the upper trough aids in lift. Latest run of the
HRRR is targeting the Sierra Nevada in Fresno County for the best
chance of thunderstorm development this afternoon. The trough is
expected to move overhead tomorrow, bringing down afternoon highs
generally 3-6 degrees below average for mid to late July as upper
heights fall. Hi resolution guidance is looking more bearish for
convective coverage tomorrow, so storms will likely be confined
north of Fresno County if they do form. Breezy conditions along
the west side hills may develop by afternoon as the trough moves
inland as well.
Slightly cooler again Thursday as the closed low weakens and opens
into an open wave and moves northeast out of the area. Thereafter,
upper heights are projected to rise slowly into the weekend, which
should result in a modest warming trend. Blended model guidance is
suggesting the return of triple digit heat for most of the
forecast area by next Monday.
Isolated thunderstorms along the Sierra Crest through 03z
Wednesday. Otherwise, VFR conditions will prevail elsewhere
across the central California interior during the next 24 hours.
.AIR QUALITY ISSUES...
The level of certainty for days 1 and 2 is high.
The level of certainty for days 3 through 7 is medium.
Certainty levels include low...medium...and high. Please visit
www.weather.gov/hnx/certainty.html for additional information
an/or to provide feedback.
AREA FORECAST DISCUSSION
National Weather Service Wilmington OH
944 PM EDT Tue Jul 21 2020
Upper level energy will cross the region this afternoon and help
lift a warm front northeast of the I-70 corridor this evening. A
more vigorous shortwave will approach from the west overnight,
spreading showers and thunderstorms across the region from
northwest to southeast. This upper level wave and eventual
surface cold front will lay out west to east through northern
Kentucky Wednesday night and early Thursday, with a lull in
activity expected from Thursday through the weekend.
.NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
The convection that was primarily diurnally-driven/initiating
off of a weakening outflow boundary which pivoted southeast
through the ILN FA has since waned locally, setting up an
environment with a fairly stable boundary layer -- especially
near the I-70 corridor and points northward. This is well-
reflected not only on SPC mesoanalysis page with ample
CIN positioned from near Dayton to Columbus and points north but
also by the nearly-saturated surface obs with dewpoint
depressions of only 5 degrees or so for most locales from east-
central IN through central Ohio.
With this being said, the aforementioned convectively-generated
outflow petered out from approximately Butler Co to Ross Co OH,
leaving an instability gradient oriented west-to-east across the
southern third of the local area. The evolution of this somewhat
untapped environment will need to be watched as a fairly
organized MCS approaches from the west through midnight. Short-
term hi-res data continues to handle the situation very poorly
and even the HRRR continues to play catchup with regards to the
evolving situation by several hours. Do think that the hi-res
models are also overestimating the BL CIN across the southern
third of the local area and are therefore killing the convection
too quickly as it progresses east closer to the ILN FA through
11 PM. Although some of the cloud tops have warmed over the past
hour or two -- there are still potent updrafts firing along the
leading edge (as seen on GOES-16 IR and through traditional
radar interrogation techniques). With the upstream environment
still relatively ripe for convection -- with about 1500j/kg
MLCAPE in south-central/central IN -- feel that the MCS may
continue to have episodic strengthening of updrafts on the
leading edge of an increasingly-cold pool dominated complex
through the next 2-4 hours or so. While the cloud-bearing
/deeper-layer flow would suggest more of an ENE propagation, the
complex may attempt to develop more to the southeast in the
direction of the more robust low level instby pooling across
south-central IN/southeast Indiana. This should result in better
maintenance of convection of the southern flank -- perhaps
eventually trailing to the southwest of the main MCS later in
the overnight with the strengthening of a LLJ -- as it moves
into SE IN and SW OH/northern KY by midnight. Not a clear-cut
situation by any means -- but will likely see most robust
convection focus further south with time as the northern segment
of the MCS will be running into a thermodynamically-unfavorable
environment near the I-70 corridor. Many factors at play here
but decided to introduce likely PoPs in the west/central parts
of the area between about 11 PM and 3 AM as the core of the
convective complex -- or what remains of it -- quickly pushes
east. With decent LL/effective shear and a moderately unstable
environment immediately ahead of the strongest cores, will
likely see at least /some/ severe threat evolve into the western
parts of the local area through about 1 AM or so. Beyond this,
the evolution of the mesoscale environment remains a bit more
uncertain, so will at least say that spots from near Dayton to
Wilmington to Maysville and locales further to the southwest may
see a low end severe threat evolve for several hours later this
evening. Will likely see at least a few SPSs for gusty winds by
midnight in our west/southwest. Feel that the expansion of the
MRGL risk east into the SW part of the ILN FA is well-warranted
to cover the evolving weather situation late this evening.
The MCS /eventually/ will begin suffer on account of diurnally-
induced stabilization of the environment and an outrunning of
the main convective line from the source of focus/lift. This
being said, even after the MCS moves through -- with the
remnants exiting our east by about 08z or so, additional LLJ-
induced shower activity will continue to sprout across
IN/western OH through daybreak as the main elongated S/W energy
approaches from the WNW.
Will see temps bottom out in the lower 70s area-wide.
.SHORT TERM /6 AM WEDNESDAY MORNING THROUGH WEDNESDAY NIGHT/...
Storms will spread NW to SE and linger for the bulk of the day,
keeping highs in the low to mid 80s in the north and near 90 in
the southeast which may stay dry for a better portion of the
Storms will line up on a more w-e boundary that lays out south
of the Ohio River later overnight, possibly lingering in the
evening for the eastern sections of the CWA and Scioto Valley.
Lows will be around 70 and could go lower if the cold front is
more pronounced with a northwest flow, versus the westerly shift
.LONG TERM /THURSDAY THROUGH TUESDAY/...
Mid level short wave energy will move across the lower Great
Lakes through the day on Thursday. This will help push a cold
front southeast into the upper Ohio Valley Thursday into
Thursday night, leading to an increasing chance of showers and
thunderstorms as we head through the afternoon. The front will
then stall out along the Ohio River as it weakens Thursday night
into Friday. As a result, will linger some lower chance pops
through Friday, especially across southern portions of our fa.
Highs Thursday and Friday will be in the mid to upper 80s.
Mid level ridging over the mid Mississippi Valley will try to
build a little more to the northwest through Saturday, but some
very weak mid level energy dropping down the front side of the
ridge may be enough to allow for an isolated shower or
thunderstorm Saturday afternoon. The ridge will then begin to
break down with northwest flow aloft setting up across the
region through early next week. Occasional weak short waves
dropping down from the northwest will lead to some lower end
chances for thunderstorms at times for Sunday and Monday. Highs
both days will be in the upper 80s to lower 90s. A cold front
may eventually push southeast into the region heading into
Tuesday, resulting in some better chances for showers and
thunderstorms by the end of the long term period. Highs on
Tuesday will be in the mid to upper 80s.
.AVIATION /02Z WEDNESDAY THROUGH SUNDAY/...
For the most part, VFR conditions will prevail through the
entire period for all sites. Isolated showers/storms will wane
through the first hour or so of the valid TAF period before a
decaying cluster of storms again moves through by/past 05z or
so. There is a great deal of uncertainty regarding the overnight
activity and its maintenance as it progresses eastward into the
local area between 05z-11z or so. It appears that western sites
of KCVG, KDAY, and KLUK may see the best chance of
showers/storms during this time period, but confidence is
admittedly fairly low in each site being impacted by a SHRA or
Surface winds will generally be light and variable for the
overnight period but will be convectively-determined with
various mesoscale boundaries/features persisting through the
overnight period. Winds, in general, will increase to about
8-10kts past 15z from the southwest.
Main item of interest for the daytime period will be fairly
widespread but disorganized convection developing as early as
15z. Activity will be somewhat chaotic in nature, so timing out
a particular impact at one particular terminal is difficult at
best. Decided to broad-brush a VCTS for all sites with TEMPO
TSRAs from late morning through the afternoon -- with an earlier
initiation expected for western sites of KCVG, KLUK, and KDAY.
May also see isolated activity linger past sunset/00z Thursday.
Any storm from late morning through the afternoon will be
capable of brief gusty winds and reductions to both CIGs and
VSBYs to MVFR or lower.
OUTLOOK...Thunderstorms will be possible at times through
previous forecast discussion noted the potential for heat headlines
by the weekend and given the strong signal across guidance for
aclimatological heat and humidity (this forecast again has afternoon
heat indices of 95-100 both Saturday and Sunday) it`s hard to
disagree with that assessment. Further contributing to the misery
will be the lack of relief at night with lows only dropping into the
mid 60s interior and staying in the 70s along the Lake Superior
shoreline where there will be continued downslope warming through
the night on the south wind.
As for precip chances, well that`s a bit harder to say. On the large
scale, it seems the height gradient will actually be too far north
for much precip during the day Saturday or Sunday. Any ridge rollers
would follow the height gradient across western and northern Lake
Superior. Thus, Isle Royale and the Keweenaw have the best chances
for storms Saturday and Sunday. However, with so much moisture and
instability around, it`s possible the lake breezes convect each day,
so more or less held on to the NBM`s chance POPs each day. The
better chance area-wide right now looks to be Sunday night into
Monday when a cold front drops southward across the UP. Experimental
CIPS Analog Guidance shows somewhat of a severe weather signal as
well, which makes sense with the continued high CAPE and at least
modest deep layer shear. But of course those details are difficult
to discern so far out, so stay tuned for updates.
.AVIATION...(For the 00Z TAFS through 00Z Wednesday evening)
Issued at 622 PM EDT TUE JUL 21 2020
Rain showers will continue to move through tonight with increasing
low level moisture. Will maintain MVFR conditions into tonight with
the potential for periods of IFR at KIWD. Expect all sites to lower
to IFR by early Wed morning.
.MARINE...(For the 4 PM Lake Superior forecast issuance)
Issued at 415 PM EDT TUE JUL 21 2020
Low pressure will pass south of Lake Superior tonight into Wednesday
with east to northeast winds to near 20kts. A few northeast gusts to
25 knots are possible over far western Lake Superior. These winds
will weaken and shift northerly late Wednesday in response to high
pressure building in from the west. Generally light winds will
continue into Friday as high pressure continues to dominate. South
to southwest winds could increase as high as 15 to 25 knots by
Saturday as another trough/frontal system approaches from the
Short Term forecast discussion from this afternoon is still
relevant as we move into tomorrow.
00Z TAF DISCUSSION.
Showers and thunderstorm activity should continue to wind down,
with the exception being at KCHA where a line of storms from
northern GA may move over the airport in the first couple of
hours of the TAF. Fog may once again form overnight, but certainty
on location of fog wont increase until we see where rain has
occurred this evening. Another round of storms is expected
tomorrow, but coverage should be higher than what we saw today.
/ISSUED 330 PM EDT Tue Jul 21 2020/
SHORT TERM (This Afternoon through Wednesday)...
Current water vapor imagery and RAP upper air analysis depict a
trough stretching from the Upper Mississippi River Valley
southwestward into the southern Plains with a ridge across the
western Atlantic into the southeastern U.S. This ridge has been the
main feature for the last several days. At the surface, there is a
front to the north stretching from IL to the east into the Mid-
Atlantic. This front is forecast to wash out as a more dynamic
system moves in from the northwest associated with the
aforementioned upper trough. Current radar imagery indicates
scattered convection across the area with best coverage, so far, over
the higher elevations. The cu field is expanding on visible
satellite imagery and should see the coverage of showers and storms
increase throughout the afternoon. Temperatures are in the upper 80s
to mid 90s early this afternoon with surface dewpoints in the upper
60s and lows 70s. The atmosphere remains relatively unchanged over
the last several days with modest CAPE values and little to no
shear. In this typical summertime atmosphere, cannot rule out a
strong to marginally severe storm or two with an isolated damaging
wind gust being the main threat. Most storms will remain well below
severe criteria. Additionally, PW values are in the 1.7-1.9 inch
range. Therefore, any storm that does form could produce heavy
rainfall. With the lack of any upper flow to really move the storms,
localized flash flooding could become an issue with some of these
storms. Convection should begin to diminish in coverage and
intensity around sunset but with the instability and outflow
initiated storms, precipitation could linger into the early
overnight period. Lows tonight will once again range from the upper
60s to mid 70s. Some patchy fog could be possible around sunrise,
especially for areas that experience rain later this afternoon and
The upper level trough will move across the Great Lakes and Ohio
River Valley on Wednesday. Upper level heights will fall slightly
across the area but the ridge looks to remain resilient and 500 dm
heights will still be 590 dm+. The front should remain to the north
of the forecast area throughout the short-term forecast period but
will drop southward across IL/IN/OH on Wednesday. Heating throughout
the day will yield similar instability as the past several days. The
forecast area will also be in the right entrance region of a 60-80
kt 300 mb jet that will be located across the eastern Great Lakes
into the Northeastern U.S. This will provide some weak synoptic lift
as well. This lift will be coupled with elevated PW values in the
1.8-2.0 inch range. Therefore, have a bit higher confidence in
coverage of showers/storms tomorrow afternoon. A few of these could
be strong to marginally severe with an isolated damaging wind gusts
being the main threat but most storms will remain well below severe
criteria. Additionally, these showers/storms will produce heavy
rainfall and could lead to some area of localized flash flooding.
Best chances for a strong to severe storm will be across southwest
Virgina closer to the lift and front. Highs will be slightly cooler
tomorrow but will still be in the upper 80s to low 90s for most
LONG TERM (Wednesday Night through Tuesday)...
1. Higher chances for showers/storms continues Thursday; some could
produce localized flooding and isolated damaging winds and hail.
2. Turning hotter and drier this weekend through early next week
with only scattered afternoon/evening thunderstorms.
Wednesday Night Through Thursday Night...
As the mid/upper shortwave over the Great Lakes translates E Wed
night, the associated surface low will move along the borders of
upstate NY and SE Ontario and drag a cold front into the lower OH
valley and central Appalachians before the boundary briefly stalls.
This front and upper level support from the shortwave passing to the
N will keep chance PoPs for showers/storms Wed night, but coverage
will not be very widespread due to nocturnal cooling. Coverage of
convection will rapidly increase from mid morning through the
afternoon Thursday as a secondary shortwave digs into the NE U.S.
and begins to push the front slowly southward through the S
Appalachians region. The strongest forcing is tied to the
aforementioned shortwave over the NE, but some confluence aloft and
mid level vorticity, likely associated with old convection, will
work with daytime instability, lack of capping, and convergence along
the boundary to generate widespread convection. Seasonably high
values of instability are expected with MLCAPE of 1500 to just over
2000 J/Kg and LI`s of -6 to -8, but deep layer shear is weak at only
10-20 kts in the 0-6 Km layer. The highest shear is over NE TN and
SW VA. SPC Day 3 outlook just outlines general thunder, but this
environment supports localized severe weather with damaging winds
the main hazard, but slightly lowering WBZ heights could also
support localized hail. DCAPE is a little low mostly in the 500-700
J/Kg range, so any wind damage should be spotty. The bigger impact
will be locally heavy rain from convection since the weak flow will
lead to slow storm motions and backbuilding, and PWATs of 1.8 to
just over 2 inches supports efficient rainfall rates. Any areas that
see heavy rain on Wed will be the most primed for localized
flooding. Convection will wane after dark Thurs night, but with the
front crossing the region, kept chance/slight chance PoPs through
the night. Temps will be seasonable this period (upper 80`s/low 90`s
for highs and upper 60`s/low 70`s for lows).
Friday and Friday Night...
The cold front should push S of the region through the morning on
Fri as NW flow develops over the eastern CONUS in the wake of the
stronger NE U.S. shortwave, and surface high pressure moves into the
Great Lakes. This will bring drier air into the region, but very
limited cooling. There will be afternoon instability again, but took
PoPs down significantly from the NBM with chance across the region
Fri since the front will be moving S. Went dry Fri night with
nocturnal cooling and drier air finally winning out. Highs will
still reach the upper 80`s/low 90`s with lows in the upper 60`s/low
Saturday through Tuesday...
This period still looks to have building heat, but deterministic and
ensemble guidance have all trended slightly farther W with the
center of the ~594 DM H5 high this weekend, which will then begin to
retrograde toward the Rockies by Tues as shortwave energy dives into
the Great Lakes and brings back a NW flow pattern by midweek. This
means that highs in the low 90`s with perhaps a few mid 90`s are
most reasonable during this period with chance PoPs each afternoon
as diurnal instability interacts with weak perturbations rotating
around the eastern periphery of the high.
.PRELIMINARY POINT TEMPS/POPS...
Chattanooga Airport, TN 74 93 73 93 73 / 30 40 20 60 20
Knoxville McGhee Tyson Airport, TN 73 91 73 92 72 / 30 40 30 70 30
Oak Ridge, TN 73 91 72 92 72 / 30 40 30 70 30
Tri Cities Airport, TN 69 89 69 87 68 / 20 40 40 70 40
Area Forecast Discussion
National Weather Service Nashville TN
708 PM CDT Tue Jul 21 2020
FOR EVENING DISCUSSION.
Will opt to delay the decreasing pops as isol to sct coverage
persists. Hrrr wants to hold on to the activity into the evening
and given the current cape values this looks believable. This
evenings upper divergence appears to shift over eastern areas by
03Z and so the best shot of convection holding on is across that
area. Otw, overnight lows still look acceptable. No other changes.
00Z TAF DISCUSSION.
Isolated to scattered convection again in place across middle TN
this afternoon. A weakening shortwave axis will be on approach
tomorrow and thus, afternoon coverage will be greater as compared
with today`s activity. Otw, tover values are running positive this
afternoon and so for the overnight, just light fog will again be
possible, particularly for areas where rainfall occurred earlier.
Area Forecast Discussion
National Weather Service Blacksburg VA
906 PM EDT Tue Jul 21 2020
An upper ridge across the southern U.S. will maintain itself
into Thursday before a front arrives late in the week.
.NEAR TERM /THROUGH WEDNESDAY/...
As of 905 PM EDT Tuesday...
Widely scattered storms continue under an axis of DCAPE values
in excess of 1000 J/KG, so locally damaging winds are still
possible for the reminder of the evening. 23Z HRRR has a good
handle on the current location of convection and keeps storms
across the VA southside until around midnight-1 AM EDT. Adjusted
POPs up in this area.
There will be a weakness in the upper ridge as a shortwave
passes into the Ohio Valley late tonight. This may help a
complex of storms move into WV by daybreak, but the model
consensus is to keep this west of our area through 12Z (8 AM
With a stagnant airmass in place, followed persistence for lows
and bumped up lows at ROA closer to values experienced Tuesday
Wednesday appears somewhat similar to this afternoon as storms
fire up noon or just before. Difference is a slight cooling
aloft with trough moving across the upper Ohio Valley. SPC has a
slight risk for severe north of U.S. 60, as better upper
support/shear push across northern WV/VA, and density from
storms may sink the convective system south toward Lynchburg to
Roanoke/Lexington by mid afternoon. Overall storm coverage looks
more numerous in the Shenandoah Valley and mountains, with less
southside VA/NC piedmont.
Highs Wednesday will be in the mid 80s to around 90 west, to
mid 90s east. The exception will be the highest terrain of SW
VA/NW NC with upper 70s to lower 80s above 3500 feet. See
climate section below for records.
Forecast confidence is average for storm location/coverage this
evening and again Wednesday, but high on temperatures.
.SHORT TERM /WEDNESDAY NIGHT THROUGH FRIDAY NIGHT/...
As of 200 PM EDT Tuesday...
An area of low pressure will cross over Maine in association with a
passing upper level trough during Wednesday night into Thursday. It
will drag a cold front toward Ohio and Pennsylvania by Thursday
afternoon, which should spark another round of showers and
thunderstorms across the Mid Atlantic. There remains plenty of
instability ahead of this front based on the ample heat and
humidity. Thus, the Storm Prediction Center has highlighted a
marginal risk of severe weather mainly along and east of the Blue
Ridge. The record consecutive streak of 22 days for hitting at least
90 degrees (June 23 through July 14 in 1966) at Roanoke will be
toast by midday Thursday as we reach day 23 of this current streak.
The cold front should slowly sag southward through central Virginia
by Thursday night into Friday, which will push the focus for
convection toward the Virginia and North Carolina border during
Friday afternoon. Meanwhile, the impressive upper level ridge
responsible for all the heat will start to shift westward toward the
Plains as the upper level trough digs further across the Northeast.
Slightly drier air could infiltrate from the northwest by Friday
night as high pressure takes control over New York.
.LONG TERM /SATURDAY THROUGH TUESDAY/...
As of 200 PM EDT Tuesday...
The cold front should push further southward across North Carolina
on Saturday and eventually head offshore toward the latter half of
this weekend. Confidence is increasing for shower and thunderstorm
chances to diminish by Sunday based on better agreement between the
ECMWF and GFS models. Still, a chance will be left across the
northwest North Carolina mountains as orographical lift may spark
isolated convection. Temperatures could be a little cooler for
Saturday compared to earlier in the week, but this slight reprieve
will be brief as the influence of the upper level trough should
fade away by Sunday.
If the record streak of 90-degree days at Roanoke is not halted by
Saturday, it will continue well into next week as heights aloft rise
during Sunday and Monday. Consequently, the heat and humidity should
begin to creep up again. The potential for showers and thunderstorms
could tick up by Monday afternoon west of the Blue Ridge as the next
cold front and its associated upper level trough enters the Ohio
River Valley. Convection chances increase further on Tuesday as the
trough digs southward and pushes the cold front toward the
Appalachian Mountains. The slower ECWMF solution was favored for the
latter part of this forecast as the GFS seems too fast given this
.AVIATION /01Z WEDNESDAY THROUGH SUNDAY/...
As of 645 PM EDT Tuesday...
Scattered showers and thunderstorms across portions of southwest
Virginia will gradually diminish this evening. Kept VCTS for all
TAF sites except KLYH until 02-03Z (10-11 PM EDT). Once the
storms diminish VFR conditions will prevail for most TAF sites
overnight, but MVFR fog, or worse, is possible at KLWB before
Expect better coverage of thunderstorms on Wednesday, especially
after 18Z (2 PM EDT), and included VCTS at all TAF sites, as it
is too early to pinpoint which locations will be directly
impacted. These storms will have the potential to produce gusty
winds and heavy rain and should they move over a TAF site, a
short period of IFR conditions are possible.
Extended Aviation Discussion...
Overall mainly VFR weather to continue through most of the
week. We will continue to have afternoon and early evening
scattered storms through the extended period. Thus periods of
sub-VFR are possible in storms, and potential for late
night/early morning fog at LWB/BCB and perhaps other locations
depending on where there is daily significant rainfall.
Record temps for today at climate locations:
Site High/Year Warm Low/Year
BLUEFIELD 94 1934 78 1934
DANVILLE 102 1977 75 2015
LYNCHBURG 103 1926 80 1930
ROANOKE 104 1926 75 2008
BLACKSBURG 99 1926 74 1934
Area Forecast Discussion
National Weather Service Sacramento CA
331 PM PDT Tue Jul 21 2020
Gradual cooling trend this week. A few late day thunderstorms are
expected over the mountains. Keep an eye to the sky if in the
mountains. Southwest breezes continue near the Delta with locally
breezy conditions elsewhere.
Visible satellite imagery shows some convective cumulus clouds
beginning to bubble up along the Sierra crest this early
afternoon. Additionally, plenty of mid/high clouds continue to
stream onshore the central California coast, providing some
filtered sunshine to the San Joaquin and southern Sacramento
Valleys. Main story for today will be the potential for
afternoon/evening convection in the higher elevations across
portions of central and northern California. The latest HRRR and
3-km NAM are generally keeping the convection along/east of the
Sierra crest as well as near the peaks of the coastal ranges, but
will closely monitor radar as the afternoon progresses.
Upper level low centered off the northern California coast will
push inland over the next 24-48 hours. This upper low along with
moisture in the mid/upper levels will provided a renewed
opportunity for showers and isolated thunderstorms for the
mountains on Wednesday. Additionally, a welcomed relief from the
triple digits will be finally realized in the northern Sacramento
Valley by tomorrow as onshore flow continues to ramp up and
temperatures moderate closer to seasonal normals. Trough will sag
over the West Coast into the second half of the week with highs
near or even slightly below normal. // Rowe
.EXTENDED DISCUSSION (Saturday THROUGH Tuesday)...
Short wave energy moving through Wrn Canada on Sat should result in
a more stable atmosphere over Norcal, and little if any thunderstorm
activity in our CWA (south of Hwy 50), even though a broad weak trof
will remain over our region. Re-establishment/re-orientation of this
weak trof over Norcal could trigger isolated convection over the Nrn
Mtns/ Srn Cascades Sun/Mon. This will be followed by drier, more
stable flow over Norcal on Tue as a longer wave trof is forecast to
develop over the Ern Pac/along the Wrn Coast of NOAM by the middle
of next week. Temperatures should remain close to average on Sat,
then trending slightly warmer into early next week. This will mean a
return to triple digit temps up north and mid to upper 90s in the
Srn Sac/Nrn San Joaquin Valley and lower foothills. Onshore flow
will have an ameliorating influence on temps in the marine/Delta
influenced areas. JHM
VFR at Norcal TAF sites with some local/enroute wx concerns. First,
scattered afternoon/evening thunderstorms over Sierra between 21z
Tue-03z Wed with isolated storms over Trinity Alps/Shasta Co mtns.
Second, local areas of smoke (FU HZ) from Wildfires mainly Shasta
and Plumas Counties. Third, SWly surface wind 25g35kt through the
Carquinez Strait/Delta with marine stratus spreading into coastal
range valleys and Wrn Solano after 06z Tue with TOPS 020-025.
Stratus retreating to the coast after 18z Wed. Upper low off the
SFO Bay area coast spreading high cloud cover (BKN AOA 200) over
SAC area/SCK/MOD TAF sites through 07z Wed with conditions
becoming SKC thereafter.