Forecast Discussions mentioning any of
"HRRR" "RAP" "RUC13" "RUC" "RR" received at GSD on 04/18/20
Area Forecast Discussion
National Weather Service Boise ID
854 PM MDT Fri Apr 17 2020
.DISCUSSION...No forecast changes through midday Saturday, but the
new HRRR threatens isolated showers and/or thunderstorms in all of
eastern Oregon Saturday afternoon, i.e., earlier than our forecast.
For now the HRRR is the only model that does it so we won`t update
at this time. But if other 00Z models come in with earlier showers,
the overnight forecast package will probably include them.
Otherwise, the current forecast looks fine, with several degrees
warming expected Saturday, and a milder night Saturday night.
.AVIATION...VFR. Mostly clear, then increasing high and mid level
clouds Sat afternoon/evening. Isolated showers/thunderstorms with
MVFR and mountain obscurations expected after Saturday/21Z.
Thunderstorms could produce erratic gusts of 20-30 kt. Snow levels
above 6kft MSL. Surface winds: outside of storms, variable 8 kt or
less, becoming W to NW 5-15 kt after Saturday/18Z. Winds aloft at
10kft MSL: SW to W 5-15 kt.
Sunday Outlook...Mostly VFR. Isolated showers bringing MVFR
conditions through Sunday morning. Snow levels generally around
5500-6500 ft MSL.
SHORT TERM...Tonight through Sunday night...Another cool, clear
night under a dry and stable air mass. An upper trough will
approach the region from the northwest on Saturday, increasing
clouds from north to south through the day. Southeast Oregon and
northern portions of the w-central Idaho mountains will see
isolated showers develop by early evening with the slight chance
they develop into thunderstorms. Passage of a front Saturday
night will bring gusty winds to 25 mph. Showers will continue
across portions of southeast Oregon and southwest Idaho south
of the Snake Plain Saturday night into Sunday as the trough crosses
the area. Thunderstorms are again possible Sunday afternoon across
higher terrain surrounding the central Snake Plain. Shower activity
will diminish by Sunday evening as the trough exits eastward.
Temperatures will be slightly above normal through the weekend.
LONG TERM...Monday through Friday...High pressure over the
Pacific Northwest will help drive an upper low to our south leaving
a slight chance for shower activity through higher elevations close
to the NV border Monday afternoon. As this ridge weakens, a stronger
system will push through Wednesday creating scattered showers
through early Thursday. Drier conditions expected Friday as we
sit in between systems. Temperatures swill it above normal through
mid week then drop to near normal behind the frontal system.
PREV SHORT TERM...DG
PREV LONG TERM....JC
Area Forecast Discussion
National Weather Service Denver/Boulder CO
840 PM MDT Fri Apr 17 2020
Issued at 830 PM MDT Fri Apr 17 2020
Have updated the grids this evening to account for patchy fog the
appears be developing, primarily along the South Platte River
Basin. Latest HRRR shows very shallow moisture lingering across
this area most of the night. Otherwise, no other adjustments
.SHORT TERM...(This evening through Saturday)
Issued at 214 PM MDT Fri Apr 17 2020
Water vapor imagery showing very dry and subsident
airmass across northeast Colorado this afternoon with warming
temperatures and melting snow. Temperatures have risen into the
upper 40s/lower 50s with bare ground, while snow covered areas
temperatures struggling still in the 30s. For tonight, mainly
clear skies but will see some increase in middle and high level
moisture in advance of storm system over the desert southwest.
For Saturday, a closed low now over Southern California will
gradually weaken and move across Southern Colorado Saturday
afternoon with some weak to moderate QG ascent with this system.
However, the best ascent will be across Southern Colorado.
Coupled with some moisture and steepening lapse rates will result
in a good chance for mountain showers and possibly a few
thunderstorms as well in the afternoon. Some of these showers will
also bleed over the adjacent plains and Palmer Divide. Surface
based CAPE values up around 800j/kg so don`t really expect any
severe storms. Temperatures will continue to warm into the 50s
except 40s over snow covered areas. Current forecasts has things
in good shape so only minor changes.
.LONG TERM...(Saturday night through Friday)
Issued at 214 PM MDT Fri Apr 17 2020
A weak shortwave will move across Colorado Saturday evening into
Sunday. The upper level jet will be further south bringing mostly
orographic snow to the higher mountains with the increased moisture
associated with the disturbance. As the jet pushes north over the
region lapse rates across the higher elevations will be in the 8-9
C/km range increasing showers over the mountains with a slight
chance over the plains by the evening hours on Sunday. Temperatures
will be closer to seasonal normals with highs in the lower 60s
across the plains.
For Monday, west to northwest flow will be across the area with
enough mid-level moisture for some isolated showers in the mountains
by the afternoon. Temperatures will include their upward climb with
highs reaching into the mid-60s.
For the remainder of the week an upper level trough will move across
the southern Rockies Tuesday with the best lift and moisture over
the desert southwest. Models are showing enough CAPE for some
isolated showers and storms for the plains by the afternoon/evening
hours Tuesday. Conditions will dry our slightly on Wednesday with
some mid-level ridging given the models desire to put in pops. Will
keep on the dryer side with on a slight chance across the higher
elevations to account for any diurnal showers. Temperatures will be
around seasonal normals through the week with highs registering into
the 60s to possibly lower 70s for some areas out east.
.AVIATION...(For the 00Z TAFS through 00Z Saturday evening)
Issued at 830 PM MDT Fri Apr 17 2020
MVFR vsbys a short time ago with patchy fog developing at KDEN.
Light northwest winds at this time, expected to go to drainage
later this evening which should shift the fog north and east of
KDEN. If this drainage wind does not kick in then patchy fog may
linger at KDEN and well as KBJC overnight. Outside of this, VFR
expected tonight with mainly clear skies.
Some increase in mid/high level moisture during Saturday which
may lead to a brief window for showers in the afternoon. I went
ahead and added a prob30 group to account for the showers. Could
see some brief outflow wind gusts 20-25 kts with any showers that
develop near the terminals after 21z.
Area Forecast Discussion
National Weather Service Caribou ME
915 PM EDT Fri Apr 17 2020
High pressure will build in tonight into Saturday while Low
tracks south of the region. A cold front will cross the region
later Sunday night through early Monday. Low pressure will
approach from the west Tuesday and pass through the region on
.NEAR TERM /THROUGH SATURDAY/...
9:15 pm update: Just about all of the diurnal showers have
dissipated. There could still be a flurry in spots near
Stockholm and Loring, but likely just a bit of virga. It is now
mainly clear across the FA, but still partly cloudy across
northern Aroostook County. Attention turns to low pressure
moving out of the OH Valley and into western PA as of 9 pm. The
low will track to the NY bight by 12Z Saturday spreading clouds
across the Downeast Region overnight. Made some minor
adjustments to the forecast based on the current and expected
conditions, but overall the changes were mostly minor.
The sfc trof swinging across the region this afternoon w/rain-
snow showers. Steep llvl lapse rates with cold air aloft has
allowed for instability. Some of the activity included gusty
winds and brief heavy rain/snow as was evident in
Frenchville(KFVE) this afternoon. The latest HRRR along the NMM
guidance did well in coverage and showing the potential for
heavier activity. The sfc trof will be passing off to the east
w/winds dropping off and clearing skies. Temps will drop off
sharply after sunset w/the winds dying off. Some areas back
across the Allagash and St. John Valley could see mid to upper
teens by 6 AM Saturday.
High pres builds across the region on Saturday while low pres
passing by to our s. The northern fringe of the precip shield
could skirt the coastal region, especially along the outer
islands on Saturday. The latest NAM/GFS show some light precip
along the coast. The ECMWF matches close and has been consistent
with this setup. Decided to stay w/the midnight crew`s thinking
of 20-30% percent chances for some light rain along the coast.
Further n, dry and less clouds w/miler temperatures. Daytime
temps for Saturday should reach near normals.
.SHORT TERM /SATURDAY NIGHT THROUGH MONDAY/...
Ridge will build quickly behind exiting low pressure, with SW
flow restricting much cool NW wind in its wake. This advects
warm air into the region as well as some clearing remaining
through the early afternoon. Temps warm into the upper 40s
across the north with 50s across central and Downeast areas.
Clouds increase during the afternoon as a weakening cold front
approaches from the west. Expect rain showers to arrive during
the evening across the north, and pass through Downeast after
midnight. As temperatures drop with the passage, some of the
rain showers will change to light snow showers across the north
up to the early Monday morning hours. Expect QPF to remain low
as guidance continues to suggest weakening with its translation
Clouds will begin to clear Monday afternoon behind the front,
tho temperatures will be cooler than Sunday as winds get a
little breezy w/ mixing out of the NW.
.LONG TERM /MONDAY NIGHT THROUGH FRIDAY/...
GFS and Canadian continue to keep a mid-Atlantic storm system
tracking outside the Gulf of Maine Tuesday, with the ECMWF now
agreeing. As a result, have trended PoPs down earlier along the
Downeast coast. Tuesday will remain mostly dry as another system
Low pressure tracking along the southern tier of Canadian
Provinces will gain energy as a jet noses in from the NW.
Intensification looks to take place over the eastern Great Lakes
with the system becoming vertically stacked over central
Quebec. There remains the differences of timing and location of
this low, which will determine if Maine sees its precip from a
frontal passage or the low tracking through the state. A more
progressive frontal passage would be warm, resulting in mostly
rain, while a low track across the state would yield cooling
conditions across the north. This could provide some mention of
snow, particularly on the back side as cool air filters around
Either case supports a push of cool air that could support a
rain/snow mix as the system tracks to the east, so have kept a
chance in the forecast, mainly from the Central Highlands north.
PoPs decrease Wednesday night, with a few lingering snow showers
possible across the north. Zonal flow takes over briefly with
widespread westerly motion across New England as a high exits
the mid-Atlantic coast. The next precip comes as a storm carries
up the return flow towards the Northeast. Broad low pressure
develops, with a few lows approaching, one towards the Gulf of
Maine, and another across the Ohio Valley. While GFS and ECMWF
both show these systems developing, timing and placement are not
in high confidence at this time and will keep forecasts below
.AVIATION /01Z SATURDAY THROUGH WEDNESDAY/...
NEAR TERM: VFR through Saturday. The caveat is KBHB where MVFR
cigs are possible w/low pres passing by to the south. MVFR
ceilings were introduced with the 00z Tafs starting around 18Z.
Sat Night - Sun...VFR all terminals with the exception of BHB on
Saturday night where MVFR cigs are possible.
Sun night...MVFR northern terminals in low cigs and -shrasn. SW
winds around 5-10kt.
Mon through Mon night...VFR under high pressure all terminals.
NW winds 5-10kts gusting to 15-20kts in the afternoon.
Tue - Tues Night...Becoming MVFR in the afternoon. S winds
5-10kts gusting to around 20kts near BGR and BHB in the
afternoon. Rain showers arriving in the afternoon will change to
a mix of rain and snow across northern terminals, occasional IFR
will be possible.
Wed...MVFR/IFR will improve towards VFR late Wednesday as precip
tapers west to east. SW winds becoming W with gusts approaching
25 kt during the afternoon.
NEAR TERM: Extended the SCA until 10 pm EDT this evening.
Still getting gusts to low end SCA. The wind will slowly
diminish tonight as high pressure builds into the region.
Conditions are expected to be below SCA levels later this
evening and through the day Saturday.
SHORT TERM: Below SCA criteria is expected Sunday with winds
increasing towards SCA in the evening and early overnight hours.
Waves will respond to a SW fetch, becoming highest along the
Washington County coast. Winds and waves again fall below SCA
Monday afternoon through Tuesday morning. Another SCA may be
needed Tuesday into Wednesday as a storm system approaches from
MARINE...Small Craft Advisory until 10 PM EDT this evening for ANZ050-
AREA FORECAST DISCUSSION
National Weather Service Jackson KY
808 PM EDT Fri Apr 17 2020
Issued at 808 PM EDT FRI APR 17 2020
An outflow from the rain across northern Kentucky has shifted
southward through the evening hours with a sharp temperature drop
from the low 70s into the 50s being seen behind this boundary.
Meanwhile, rain continues to impact areas along or north of I-64,
and this rain should start to spread southward over the next few
hours. Wind gusts have subsided so the lake wind advisory and SPS
were allowed to expire. The only changes was to tighten up the
temperature gradient through tonight as that outflow moves south
and to speed up the exit of precipitation late tonight.
.SHORT TERM...(This evening through Saturday night)
Issued at 455 PM EDT FRI APR 17 2020
20z sfc analysis shows a healthy area of low pressure moving into
the Ohio Valley with a warm front lifting northeast through
Kentucky. A cold front trails this low and is poised to enter
western Kentucky shortly. This setup led to an impressive
gradient in temperatures across the CWA with upper 50s found north
of the boundary and low 70s seen to the south. The southern
warmth was enhanced by gusty southwest winds (up to 35 mph) -
particularly this afternoon. Additionally, the clouds thinned and
were mostly absent this afternoon south of the front and ample
sunshine certainly contributed to the temperature climb.
Meanwhile, dewpoints varied from the mid 40s north (and closer to
the pcpn associated with the sfc low) to the upper 20s and lower
30s in the warm sector. The higher temperatures and lower
dewpoints also dried out much of the southeast (to around or just
below 25 percent) this afternoon with the gusty winds making for
a fire weather concern throughout the latter part of the day. The
higher winds have prompted the need for a continuing of the NPW
for Lake Winds and also an SPS for the higher wind gusts (and low
RH) elsewhere - through 8 pm.
The models are in good agreement aloft through the short term
portion of the forecast. They all depict a fairly sharp, but open,
trough pressing southeast deeper into the Ohio Valley with some
significant height falls and energy. This feature quickly sweeps
through eastern Kentucky tonight followed by fast northwest flow.
The pattern will then become more zonal into Saturday night with
additional, but much weaker, energy heading this way from the
Southern Plains for Sunday. Given the limited model spread have
favored the blend as reflected by the NBM with a strong lean on
details for the fropa tonight from the HRRR and NAM12.
Sensible weather will feature a rude end to the sunshine and
warmth that much of eastern Kentucky enjoyed this afternoon as a
cold front presses through the CWA tonight. This will mean soaking
rain showers passing through most of the area overnight with the
winds settling after dark - switching northwest as the front
passes. Cannot rule out a rumble of thunder with the front, as
well but the instability appears too limited at this time to
include in the grids. Colder air follows with readings back down
in the mid to upper 30s by daybreak. High pressure then moves
overhead for the first part of the weekend, though, despite
plenty of sunshine, highs will only be in the mid to upper 50s on
Saturday. The high shifts off to the east later that night and
should set up a decent ridge to valley split into Sunday morning -
just ahead of return flow and a potential for showers from the
southwest. With the temperature split that night some frost can be
anticipated in the valleys, as well.
Mainly adjusted temperatures on Saturday night for the ridge to
valley differences. As for PoPs, tweaked them more toward the
NAM12 late tonight into Saturday morning mainly raising and
lingering them longer.
.LONG TERM...(Sunday through Friday)
Issued at 300 PM EDT FRI APR 17 2020
The extended period will likely feature two periods of wet and
active weather and an extended period of dry weather in between.
The first round of rain is on track for Sunday and Sunday night,
as a large area of low pressure moves across the Tennessee valley
region. The rain should first begin moving into our southwestern
counties early Saturday morning, and should be exiting the area by
early Monday morning. Some of the model data is trying to bring a
bit of precip into eastern Kentucky Monday night through Tuesday,
but due to model uncertainty, decided to go dry for that time
frame for now. We should see dry weather from early Monday through
early Monday afternoon. After that, it looks like we will see
another round of rain from late Wednesday through the end of the
week. This second round of rain would be in response to another
large area of low pressure that is currently progged to move
across the Ohio and Tennessee valley regions. Instability is
questionable enough that thunder was left out of the forecast for
now. Temperatures will max out mainly in the 60s each day, with
nightly lows only falling into the 40s and 50s for the majority of
.AVIATION...(For the 00Z TAFS through 00Z Saturday evening)
ISSUED AT 808 PM EDT FRI APR 17 2020
An outflow boundary continues to slowly push southward this
evening with winds shifting from the southwest to the west or
northwest. This boundary will continue to sag southward tonight
with winds continuing to shift to the west or northwest across
most TAF sites. Rain will also spread southward overnight with
ceilings coming down to low end MVFR. Higher ridgetops are more
likely to end up in IFR conditions after 06z. These low ceilings
will continue through dawn before eroding by early Saturday
Area Forecast Discussion
National Weather Service Lake Charles LA
925 PM CDT Fri Apr 17 2020
Updated POP/wx grids earlier to account for increasing convection
over the region (especially the coastal waters), which seems to be
being handled well by various high res guidance. However
convection has appeared to stall somewhat over the nwrn zones,
likely due to mid/low-level capping as noted in the 00z KLCH
soundings. However the approach/passage of our next shortwave
aloft and sfc cold front could help erode the cap and allow for
further convection later this evening. Other thermodynamic
parameters in the sounding do point to the potential for some
severe weather, primarily hail/damaging wind gusts.
Elsewhere, inherited grids/zones look in good shape as is per
Update out shortly.
.PREV DISCUSSION... /issued 628 PM CDT Fri Apr 17 2020/
A cold front is expected to push through the area late this
evening and through the overnight hours. Ceilings will lower to
MVFR then IFR during this timeframe. Thunderstorms currently
developing ahead of the front will lift northeastward through the
evening with VCTS possible at KAEX/KBPT/KLCH. Winds behind the
front will shift to northeasterly behind the front early Saturday
morning and will remain out of the northeast into the afternoon
hours. IFR ceilings should gradually mix up to MVFR by mid to late
morning and remain MVFR through most of the afternoon.
PREV DISCUSSION... /issued 332 PM CDT Fri Apr 17 2020/
Wx map shows cool front across C and NE TX, with a warm and more
humid airmass ahead of the front this afternoon. Across our
region, temperatures are in the upper 70s to lower 80s with
dewpoints ranging through the 60s.
The upper air pattern expected to remain fairly zonal over the
weekend into next week, which will keep our region under the
influence of rapidly changing weather conditions.
The first shortwave and cool front tonight. Latest blended & HRRR
guidance shows showers and isolated thunderstorms to pick up late
this afternoon and evening across Inland SE TX/C LA with the
approach of the cool front. SPC has just outlined area in Marginal
Risk of severe thunderstorms, with a few strong to briefly severe
storms anticipated. Primary severe threat will be damaging wind
gusts. Highest rain chances of 40-50% should remain north of the
U.S. 190 corridor.
The second shortwave and northward moving warm front expected
Saturday and Saturday afternoon. This will enhance moisture and
lift accompanied by the weak frontal boundary moving back
northward. With this will be increased chances of showers and
thunderstorms. SPC has our region in a Marginal Risk of severe
weather during this time period, with a primary threat of hail if
the storms reach severe limits.
The third and most significant upper level shortwave will be
moving eastward across the Gulf states Sunday and Sunday night.
With this will be a well defined sfc low over the ArkLaTex region
moving eastward as well. While specific severe weather parameters
will have to fall into place, preliminary forecasts show a
significant threat of severe weather Sunday afternoon into the
early evening hours. For this, SPC has outlined the region along
and north of I-10 in an Enhanced Risk of severe weather, Slight
Risk south of I-10 into the coastal waters. Again, 48 hours out,
specifics are a little uncertain, but appears to be trending for
an MCS or individual supercell development. This would bring all
modes of severe weather possible (tornadoes, damaging wind, and
For Monday through early Wednesday, tranquil weather expected
behind the departing storm system. However, another significant
storm system is possible late Wednesday night into Thursday across
the southern U.S. but too far out for specifics.
A weak cold front will bring a brief period of light offshore
flow tonight into early Saturday morning. The front is expected to
return north as a warm front Saturday afternoon and evening,
allowing onshore flow to resume through Sunday. A Pacific cold
front will move across on Sunday night, with a chance of strong to
possibly severe thunderstorms ahead and along the front. Expect
brief offshore flow for Monday, with onshore flow resuming for
Tuesday. The next chance of showers and thunderstorms is expected
.PRELIMINARY POINT TEMPS/POPS...
AEX 58 75 63 80 / 40 40 50 90
LCH 65 79 69 82 / 30 50 30 70
LFT 66 80 69 84 / 30 40 30 70
BPT 65 77 69 82 / 40 50 30 70
Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
851 PM CDT Fri Apr 17 2020
851 PM CDT
Relatively quiet weather across the area this evening, with weak
surface high pressure ridge drifting southeast through the cwa.
Lingering low clouds across the southeastern counties have just
about moved out of the area, while remnants of high-based open
celled stratocu deck continues to diminish in coverage to the
Only concern tonight is with the potential for some fog or low
stratus development, particularly across the southern portions of
the forecast area withing the light/variable wind regime of the
surface ridge. Combination of these light winds, clearing skies
and surface dew point spreads already in the 2-4 degree range
would suggest at least some fog potential there. Farther north of
the ridge axis, winds have turned southwest and will increase
above the surface overnight which would allow better mixing in the
top of the boundary layer. Model guidance continues to favor areas
south of the Illinois and Kankakee rivers for the best probability
of fog/low clouds overnight, in line with our going forecast.
Overall, going forecast appears to be on track at mid-evening,
with no significant changes needed at this time.
248 PM CDT
Points of note for the short term forecast are the possibility of
spotty graupel showers through early this evening over northern
Illinois, potential for fog tonight, and wind gusts to 40-45 mph
for north of I-80 on Saturday afternoon.
Snow cover is quickly melting across the area with some locations
seeing 3-4 inches of depth at 10 a.m. already vanished despite
cloud cover, showing the influence of warm ground underneath and
solar radiation penetration through the clouds. Satellite water
vapor imagery indicates a slightly positively titled northern
stream upper wave across Wisconsin, and this brings with it a -31C
500mb cold pocket. This cold pocket combined with residual
low-level moisture and convergence across southeast Wisconsin and
northern Illinois could spark isolated late afternoon showers.
The HRRR has been consistently indicating this, and despite it
trying at times to convect too readily, the soundings do show a
low-level CAPE up to 25-50 J/kg. There already has been some
light graupel reported with a shower in Round Lake, IL earlier
this afternoon. Don`t foresee any of this being a big deal, but
sounding profiles do support graupel potential under any showers
that can develop. Any activity should fade early evening.
While much of the clouds upstream are close-celled, incoming
subsidence behind the aforementioned upper wave should result in
clearing spreading in from the northwest. This means some
locations central and southeast CWA would clear pretty late in the
day. These locations already have temperature-dew point spreads
less than 7 degrees, and the far southeast CWA even has cloud
bases under 700 ft indicating ample boundary layer moisture. This
presents concerns for fog, possibly even dense in places,
especially with any added moisture from melt/sublimation. Guidance
hydrolapses generally lean a little more toward stratus but the
current boundary layer conditions and with clearing expectations
do warrant some fog mention in the forecast and messaging.
Southwest winds will be on the increase late tonight, especially
at the top of the boundary layer. This would indicate that any fog
that does develop may ease some toward daybreak, limiting
Saturday will see a deep surface low of 986 mb track across
northern/central Ontario. This will steer its warm sector over
the area and fairly sharp pressure falls during the day. This will
drive sustained southwest winds approaching 20 kt for north of
I-80 in the afternoon. Gust magnitude has potential to be
maximized given an expected sunny sky and being in the warm
sector, with surface temperatures to climb well through the 50s
and possibly even 60. If sufficient mixing can happen, some gusts
to 45 mph in the north would materialize given the forecast
vertical profiles on all available guidance. Will message gusts of
40+ mph in the north and 30+ in the south. Would commonly be more
of a fire weather concern this time of year, but fuels/ground
moisture and even boundary layer relative humidity are expected to
be high enough not to warrant an elevated threat.
348 PM CDT
Saturday night through Friday...
After a windy day on Saturday, winds should gradually diminish
Saturday night as the atmosphere decouples. Winds could remain
fairly breezy overnight ahead of an advancing weak cold front,
which will also help keep low temperatures in the 40s across the
Have maintained slight chances for precip along the front Sunday,
mainly across our far southeast, although there is the potential
for a dry frontal passage, as indicated by the ECMWF. Quiet
weather remains on track through the beginning of the work week
with dry conditions and near seasonable temps are expected with
highs in the 50s to low 60s across the area. This will be a
welcome sight as we exit this fairly long stretch of below normal
temps. Highs near the lakeshore will be slightly cooler any days
with onshore flow, including Sunday and Tuesday where temps will
be closer to the upper 40s and low 50s.
Midweek and beyond turns more unsettled bringing multiple chances
for precipitation as several waves eject off the Rockies and track
across the area. Out ahead of the first wave, temperatures on
Wednesday could soar into the upper 60s to near 70, as noted in
the previous discussion. The current forecast remains on the lower
end of guidance, however, so this may need to be increased in
later forecasts if the trend continues. It is too soon to narrow
down specifics for any of these days given the variability of the
timing and placement of each individual shortwave, but there is
some support for thunder potential with the Wednesday and Thursday
systems. In collaboration with neighboring offices have held off
on a formal thunder mention with this forecast package given a
weaker thunder signal in forecast soundings.
For the 00Z TAFs...
Aviation forecast concerns:
* Potential for fog/low stratus development tonight, primarily
south of the Chicago terminals.
* Strong southwest winds Saturday with gusts 35-40 kts.
Weak surface high pressure ridge was transiting the area early
this evening, with extensive area of MVFR ceilings now exiting the
Chicago terminals. Winds backing west-southwest as the surface
ridge moves through and will remain from that direction
overnight. Lingering scattered/broken VFR layers will persist this
evening, gradually thinning and dissipating late. Earlier
precipitation and melting of wet snow cover, combined with cloud
cover and modest winds through the day, have left the boundary
layer somewhat moist. Shallow mixing near/in the surface ridge
axis tonight may support the development of fog and/or low stratus
as clouds scatter and temps fall off after sunset. Winds just
above the boundary layer increase to 20-25 kt beyond this evening,
which especially across northern parts of the forecast area may
preclude extensive fog development but could potentially result
in some stratus development. High-res numerical model guidance
generally focuses the greatest potential for such fog or stratus
development from the southwest Chicago suburbs into east central
IL and northern IN overnight, with GYY the most likely of our
terminals (or nearby) to potentially see stratus or fog. Have not
included a vis restriction or ceiling at GYY as confidence is
fairly low, and other terminals have lower probability of such
restrictions. Will continue to monitor satellite and surface obs
trends for development this evening.
On Saturday the surface high pressure moves south of the area
across the lower Missouri and Ohio valleys, while fairly deep
surface low pressure passes north of the Great Lakes. This sets up
a strengthening surface pressure gradient across the western
Great Lakes region, producing strong southwest winds across the
forecast area. As diurnal mixing deepens into the midday and
afternoon hours, forecast soundings indicate the potential for
35-40 kt wind gusts to mix to the surface. The boundary layer will
likely remain fairly well mixed into the evening hours as well, so
gusts will likely persist through/beyond sunset though with a
decrease in magnitude as the mixed layer becomes more shallow.
Above the boundary layer however, winds are expected to increase
into the 40-45 kt range above 1500-2000 ft agl. This could be a
potential LLWS situation if the surface winds/gusts decouple more
Saturday to 10 PM Saturday.
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Area Forecast Discussion
National Weather Service Las Vegas NV
822 PM PDT Fri Apr 17 2020
.SYNOPSIS...A low pressure system continues to slide across the
area, with the area most favorable for afternoon showers and
storms shifting to southern Nevada and northwest Arizona for
Saturday. Additional weak storm systems will approach
the region next week, keeping some shower chances in place across
parts of the region and keeping any warm ups in check.
.UPDATE...Updated POPs for the overnight hours based on latest NBM
Compact closed low positioned over eastern San Bernardino
County this evening. Satellite shows cloudtops cooling east of the
low across Mohave County, and as far north as eastern Clark County
within area of ascent. This corresponds to the areas of expanding
returns on radar. Over the next 3-6 hours, this area will continue
to shift east over northwest Arizona before the low moves further
east into north central Arizona by daybreak Saturday. Area of
subsidence just behind the low has brought an end to the earlier
thunderstorms over Inyo and San Bernardino Counties. Again, as the
low moves further east that area of subsidence will overspread the
rest of eastern California and southern Nevada bringing an end to
the showers and ushering in clearing skies. The only exception will
be further west across the southern Sierra and Owens Valley where
showers associated with the next shortwave will continue overnight.
No impacts expected overnight as the threat for gusty outflow winds
up to 35 mph has passed and rainfall overnight will mainly be less
That second shortwave will cross the region on Saturday bringing
another round of showers and isolated thunderstorms mainly north of
.SHORT TERM...Through Saturday.
The focus for the short term is an upper-level low coming onshore
to the California coast and associated shower and thunderstorm
Activity is ongoing in Inyo county, with a few lightning strikes
this morning. Not expecting activity to get too crazy there today
with crowded cumulus field and early start prohibiting much in the
way of instability. Overall, scattered showers with a few
lightning strikes here and there expected across Inyo and even
arching into Esmeralda and portions of Nye county through the day.
Latest RAP indications indicate two primary pockets of
ingredients outside of Inyo county. The first is in southern San
Bernardino county late this afternoon. Modest CAPE of 750-1000
J/kg, along with 20kt of 0-6km shear could support a few showers
and storms. Downdraft CAPE of 750-1000 J/kg and dry low levels
will also support wind potential, even with weaker showers.
Outflow gusts may reach 40 MPH if anything manages to develop
A secondary area is indicated by the RAP in northern Mohave
county. Here, CAPE values of 1000-1500 J/kg will overlap with
25-35 kt of shear will be capable of supporting decent
thunderstorms. However, these parameters rely on moistening low
levels in the evening, and are far away from forcing, which
results in a low probability of occurrence. Nonetheless, it bears
watching as this area is receiving good insolation today.
For Saturday, its a general shift eastward in the area for afternoon
shower and storms, generally across parts of Clark and Lincoln
counties, with perhaps some lingering popups further wester in Nye,
Esmeralda, and Inyo counties.
.LONG TERM...Sunday through Wednesday.
After a brief lull Sunday, expecting another active period as a
trough digs into the West Coast and swings through Monday and
Tuesday. Some consistency in the timing of the main trough pushing
through, however there are numerous pieces of energy that come
together as it moves west so still some uncertainty in the details
of the evolution of the trough. In general, expect showers at
times and breezy winds the beginning of next week. Latest models
look weaker than previous runs, so the threat of widespread
significant impacts is decreasing. Temperatures should remain near
normal through the beginning of next week, then a warm up is
.AVIATION...For McCarran...Northerly winds will begin relaxing
this evening with typical drainage winds expected tonight. There
is an outside chance of some outflow making it into the valley
from distant showers in California, but direction should generally
be between 180 and 230 and gusts 20-25 kts if that happens.
Saturday will start with light and variable winds, but by the
afternoon showers and storms look to develop on the mountains and
move across the valley. These will be capable of producing some
gusty and erratic winds if they do form. Any threat of storms
should clear the area by the evening with southwest winds after
sunset. Freezing level tomorrow afternoon should be around 9500ft
MSL. Ceilings should remain above 8 Kft through the period.
For the rest of southern Nevada, northwest Arizona and southeast
California...Decreasing shower activity expected tonight, with
showers and storms developing once again tomorrow, generally from
a KIFP-KTPH line and eastward. Showers and storms may produce some
temporary wind gusts up to 35 kts. No prevailing IFR conditions
are forecast with ceilings likely remaining above 8Kft.
.SPOTTER INFORMATION STATEMENT...Spotters are encouraged to report
any significant weather or impacts according to standard operating
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