Forecast Discussions mentioning any of
"HRRR" "RAP" "RUC13" "RUC" "RR" received at GSD on 09/18/21
Area Forecast Discussion
National Weather Service State College PA
1132 PM EDT Fri Sep 17 2021
Mostly cloudy and relatively cooler late-week conditions will
break for an unseasonably warm start to the weekend. There may
be a shower or thunderstorm in spots, but most of the time will
be dry. Rain-free conditions are expected Sunday through Tuesday
with comfortable humidity. Rainfall is probable for the start
of fall, followed by a noticeable cool down into the end of next
.NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
A weakening mid level vort max lifting across western NY state
will prolong the chance of an isolated shower through late
tonight across northern Pa. HREF and latest HRRR both target
Mckean, Potter, Elk and Cameron counties for the best chance of
a shower overnight.
For most of the region, expect a warm and tranquil night under
surface ridge axis. Mostly clear skies, a calm wind and relative
high soil moisture will promote areas of valley fog. Latest
guidance continues to support lows in the 60s, some 10-15F above
.SHORT TERM /6 AM SATURDAY MORNING THROUGH SATURDAY NIGHT/...
A weak cold front will push southeast across the forecast area
Saturday afternoon and evening. The parent shortwave is progged
to pass well north of Pa, so large scale forcing should be weak.
Thus, expect only widely scattered afternoon showers along thin
ribbon of higher precipitable water values coincident with the
front. So, while an isolated brief downpour is possible, the
warm air aloft will make it difficult to generate deep
convection and thunder. This looks especially so across the
southern part of Pa, where capping inversion in model soundings
looks strong enough to suppress any convection. Will still
leave in the mention of thunder for the northeast quadrant of
the forecast area per SPC outlook, but the threat is pretty
Mixing to 850mb, where progged temps are around 15C, should
support highs of 75-80F over the Allegheny Plateau and 80-85F
Surface ridging building across Lake Erie will lead to drying
Saturday night, dropping dewpoints into the low 50s near BFD,
and nudging them down into the low 60s in the SE piedmont.
.LONG TERM /SUNDAY THROUGH FRIDAY/...
High pressure sliding across New York will lead to a pleasant
Sunday as high temperatures warm into the low 70s.
Upper ridging across the Atlantic coast should ensure dry and
warm weather early next week. However, an easterly flow
circulating around high pressure over New England should result
in slightly cooler conditions than this weekend and could
result in patchy late night stratus over the central mountains.
An examination of latest ensemble output suggests the
operational GFS is likely too fast with approaching upper trough
and cold front during midweek. Based on ECENS and GEFS timing,
have undercut NBM POPs a bit on Wednesday and focused the best
chance Wed night into early Thursday. Models support an anafront
structure, with the bulk of the rain falling behind the front
in region of fgen forcing beneath right entrance of upper jet
Med range guidance all supports cool and mainly dry conditions
late next week, as Canadian high pressure builds southeast into
.AVIATION /04Z SATURDAY THROUGH WEDNESDAY/...
For the late evening, backed off on low clouds and fog a bit,
given current trends. IPT has a low CIG so far, but no fog.
Western areas may just have enough breeze above the ground
to limit low CIGS and fog.
Isolated thunder across southwestern NY state, but these
are lifting northeast, away from BFD.
Thus just minor changes at this point.
Earlier discussion below.
For the 00Z TAF package, looking at mainly VFR conditions
this evening. Expect low clouds and fog later tonight into
Saturday morning, before conditions improve by the afternoon,
much like today.
Less chance of fog and low clouds Sunday and Monday mornings,
as the airmass dries out some.
Sun-Mon...Mainly VFR conditions.
Tues..SHRA/TSRA possible, mainly late.
Fall Equinox 2021 will be at 3:20 PM (1920 UTC) on Wednesday,
Area Forecast Discussion
National Weather Service Indianapolis IN
1110 PM EDT Fri Sep 17 2021
...Updated Aviation Discussion...
Issued at 934 PM EDT Fri Sep 17 2021
Had lower temperatures several degrees at the onset, but with dew
points in the middle to upper 60s and with stagnant airmass,
overnight lows in the 60s still look good. Also, patchy shallow fog
still looks like a good bet with mostly clear skies, very light to
calm winds and shallow moisture per HRRR soundings.
.Short Term...(This evening through Saturday night)
Issued at 259 PM EDT Fri Sep 17 2021
A warm and humid final weekend of astronomical summer is expected
across central Indiana as a fairly stagnant pattern remains in place
across the area.
This afternoon, some slightly congested cumulus is noted on
satellite imagery, while weak convective plumes occasionally show up
on radar and quickly die off in the weakly unstable and somewhat
capped airmass. In the absence of any substantial additional
support, expect this to continue through the few hours after peak
heating, but chances that anything sustains itself are low. Will
carry an unmentionable 14 pop through 00Z and then go dry overnight.
Expect that the weak scattered showers/isolated storms to our south
associated with a weak impulse on the periphery of the Nicholas
remnants over the southern Plains/western Gulf states will weaken
significantly and at least partially dissipate diurnally before
making progress into the area.
Saturday looks to be a repeat of today, with weak convective plumes
attempting to fire and failing, especially given that heights rise a
bit which may be enough to suppress even these weak plumes in the
marginally unstable airmass.
Cannot entirely rule out possibility that a very isolated shower
sustains itself briefly over some portion of the area, but even a 20
pop seems like overkill given the vast majority of the area is a
solid bet to remain dry.
Highs tomorrow should be not that dissimilar from today, in the mid
to upper 80s, as despite the slight rise in low level thicknesses
and temps, surface flow will become more northeasterly Saturday as
broad surface high pressure passes through the Great Lakes.
Tonight, in the stagnant and humid airmass, with winds likely to go
calm in many areas, and skies looking to be mostly clear, with just
a bit of high cloud, expect some patchy fog formation again,
particularly in favored rural and low lying areas. Will carry a
mention in the grids. Some shallow dense fog isn`t entirely out of
the question, but confidence is low at this time, and will likely
depend on just how clear skies get.
.Long Term...(Sunday through Friday)
Issued at 259 PM EDT Fri Sep 17 2021
Remnants of Nicholas followed by a seasonably strong cold front will
be primary concerns for the long term forecast. Sunday will begin
warm and rather humid for mid September with afternoon temps
reaching mid to upper 80s along with surface Tds in the mid 60s.
Water vapor shows upper circulation over east Texas this afternoon
with deep moisture across the south. This moisture will finally
advect north and begin to arrive across central Indiana late Sunday
afternoon and expand northward Sunday night and Monday. Models still
trying to generate some isolated convection on northern periphery of
Theta E axis Sunday afternoon so will continue a slight chance PoP
for now over the far southwest before expanding pops to entire area
Sunday night. PWATs will climb to over 2 inches by Monday as this
tropical moisture surge and upper low drifts north. These values are
near or above climatological records for mid September.
Sharp short wave trough and DPVA to move through the area Sunday
night and Monday which will likely make for a dreary and soggy start
to the work week. Efficient rain processes in this tropical airmass
could lead to 1 to possibly 2 inches of rainfall over south central
Indiana Right now models are focusing heaviest rain axis south of
our area but much uncertainty remains. Fortunately our recent dry
spell and low water levels on area streams and rivers should keep
flood threat in the low probability range.
Deeper moisture from Nicholas remnants will begin to lift northeast
as a strong cold front associated with a deepening mid level trough
approaches on Tuesday. Details on timing between these two systems
and rain chances between them still to be worked out but break in
rain possible Monday night into Tuesday before cold front moves
through later Tuesday. This front has potential for severe weather
and locally heavy rainfall given lingering PWATs in the 1.5-2" range
and strong dynamics/kinematics indicated along with favorable
afternoon peak heating timing. However, lingering cloud cover may
lead to a conditional threat within a low cape modest shear
environment. Much too early for any confidence or actionable details
but certainly worth watching and noting at this projection time.
A taste of Fall follows this front for the remainder of next week as
high pressure, dry weather and seasonable temperatures settle in.
.Aviation...(06Z TAF Issuance)
Issued at 1109 PM EDT Fri Sep 17 2021
- MVFR and worse conditions in fog 06z-12z, especially at KBMG.
Mostly clear skies, very light to calm wind and dew points in the
middle to upper 60s support MVFR and worse fog potential overnight
over at least KHUF and certainly KBMG as has been the case the last
few nights. That said, soundings showing the low level moisture to
be very shallow and thus fog will lift quickly around sunrise.
After the fog threat is over, should only see at best scattered
diurnal cu this afternoon.
Area Forecast Discussion
National Weather Service Phoenix AZ
410 PM MST Fri Sep 17 2021
A weak weather system will move through the region today into
Saturday bringing increased cloud cover, somewhat cooler
temperatures, and scattered showers and thunderstorms. Although many
locations may not see measurable rainfall, gusty winds will be
possible with any thunderstorm. Starting Sunday, dry conditions with
temperatures hovering near to slightly above average should be
common through at least the first half of next week.
An upper low just off the SoCal coast shows well on RAP mesoanalysis
and water vapor imagery. Southerly flow ahead of the low continues
to draw tropical moisture northward into the Southwest leading to
more clouds and chances for scattered showers. Moisture has
noticeably increased since this time yesterday with surface dew
points up by 30 to 40 degrees in Yuma and up 5-10 degrees in Phoenix.
Comparison between the 17/12z and 16/12z soundings shows the moistening
trend is not limited just to the lowest levels as PWATs increased
from 0.73" to 1.22" in the last 24-hours.
Current radar suggests the next uptick in activity is underway in
southwest AZ and should continue through this afternoon and evening
as atmospheric moisture increases. Isolated embedded storms will
occur this afternoon, especially south and west of Phoenix, and
along the Colorado River, where instability is amplified by daytime
heating and colder air aloft. Today`s K1Y7 soundings, objective
analysis trends, and hi-res guidance all indicate an axis of 500-
1000 j/kg of CAPE developing along the lower Colorado River area and
over mostly open desert of western AZ between I-10 and I-8. This
kind of instability will be more suitable for producing storms
capable of producing localized 40-55 mph wind gusts and higher
precipitation rates of 1.00 in/hr. Blowing dust cannot be ruled out
either with any stronger wind gusts in the aforementioned areas IVO
Gila Bend through Quartzsite. Chances for blowing dust are 30% in
that area and less than 10% elsewhere.
Activity will shift eastward through tomorrow with additional
periods of showers/storms as the upper low ejects towards into the
southern Great Basin. Timing and location of these later rounds is
uncertain but latest HREF indicates a round of showers and storms to
develop very late tonight-early Saturday morning between Phoenix and
the Colorado River, and then again Saturday afternoon and evening
across most of south-central Arizona. Any convection will retain
mentioned threats for wind gusts and rain rates.
However, moisture and dynamics are not still not supportive for
widespread high PoPs or heavy rain. PoPs oscillate between 10-25%
this afternoon through Saturday for most areas with isolated pockets
of 30-35% PoPs. NBM 24-hr QPF probabilities are equally unimpressive
with only a 25-35% chance for 0.01 inches and a 15% chance for 0.20
inches or more at nearly any point in the forecast area. Many places
will miss the rain entirely.
Decreasing heights, moisture, and clouds will keep temperatures 4
to 7 degrees cooler compared to recent days. Highs are forecast
to reach 98-102 degrees today and hover around 100 degrees through
the weekend. Temps will undergo a very modest warming trend of a
few degrees by the end of next week.
.AVIATION...Updated at 2310Z.
South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Showers and storms should remain well removed southwest of the
Phoenix area this evening such that even chances for outflow winds
are less than 10-20%. There may be substantial variability in wind
directions overnight as the tendency for east winds battles the
pressure gradient. In addition, late night/early morning elevates
showers in western AZ could create an early shift to westerly winds
mid morning. Models still suggest a 20% chance of showers and storms
approaching the metro Saturday afternoon, however confidence in
extent of TSRA still precludes any mention yet. Maintained a VCSH
mention with gusty winds as the most likely impact on operations.
Southeast California/Southwest Arizona including KIPL and KBLH:
Thunderstorms passing near KBLH through early evening will be the
greatest weather threat with erratic gusty winds. While a south
component is favored, highly variable directions are possible.
Otherwise, SCT mid/high clouds will be common through Saturday as
shower chances diminish this evening. Any further SHRA/TSRA
overnight should remain east of the terminal footprint. More
tranquil conditions at KIPL with winds likely following diurnal
trends experienced the past 24 hours.
Sunday through Thursday:
Lingering moisture across eastern Arizona on Sunday may lead to some
isolated showers and storms across the high terrain, but drying
conditions will be seen elsewhere. This drier air will then dominate
through at least the middle of next week as high pressure
strengthens across the region. High temperatures through the period
will be near to slightly above normal. Afternoon RH values will
start off mainly between 20-25% on Sunday from south-central Arizona
and areas east before lowering to 10-15% starting Monday. Overnight
RH recoveries will generally range from 25-40% through most of the
period. Winds will be fairly breezy on Sunday, but generally will
remain light while directions favor local diurnal/terrain influences
through the rest of the period.
.SPOTTER INFORMATION STATEMENT...
Spotters should follow standard reporting procedures.
Area Forecast Discussion
National Weather Service Riverton WY
519 PM MDT Fri Sep 17 2021
.SHORT TERM...(This evening through Saturday night)
Issued at 210 PM MDT Fri Sep 17 2021
Mostly clear skies with some high clouds pushing into northern
Wyoming today. Weak upper-level high pressure with mid-level
zonal flow will keep things dry through early Saturday morning.
The surface pressure gradient due to recently departed system and
low pressure in NW Montana has added in strong south winds late this
morning for northern Johnson County and some locations in the
Bighorn Basin. The winds in these location should decrease this
afternoon, while flow becomes more southwesterly with gusts over
30 mph from southwest WY into central WY and in the higher
elevations. Combined with low afternoon humidities, critical fire
weather conditions continue for much of the state. High resolution
models such as the HRRR are showing that portions of the Wind
River Basin and Bighorn Basin may not mix out though this
afternoon due to the remaining cool air, or that it may take until
8 to 9 pm for west winds to mix down. Mid-level flow becomes more
southwesterly overnight as the west coast trough digs and the
upper-level high over southern CO continues. This pattern will
actually bring moisture with weak instability from Utah into
western WY Saturday, increasing clouds and bringing a chance of
showers to far SW WY by noon. Southwest wind will increase also
around noon Saturday with areas from southern into central WY
seeing winds 20 to 25 mph with gusts over 30 mph. Low humidities
will push northeast so that critical fire weather area should
shift more toward Natrona into Johnson County, and elevated
conditions across the Bighorn Basin. Much of western WY will have
a chance of showers and thunderstorms Saturday afternoon and
early evening, with some showers possibly also moving from Dubois
area to the northeast. Latest model trends show that some areas
could see around 0.25" of rain, especially in the Wind River
Mountains. At this time, just expecting typical thunderstorms with
brief heavy rain, small hail, and gusty winds. Interesting how
this area of showers and thunderstorms has increased over the past
couple of days as the forecast time frame comes closer. Expecting
lingering showers through midnight across central WY. Early
Sunday morning will see clouds increasing across northwest WY as
the PacNW trough starts moving eastward. Saturday night may see
some breezy wind overnight, and combined with residual cloud
covers, overnight temperatures could be more mild than has
occurred with the clear skies the past few nights.
.LONG TERM...(Sunday through Friday)
Issued at 201 PM MDT Fri Sep 17 2021
Sunday will bring the NW trough into the region, with a
significant cold front sweeping through during the day Sunday. The
trough will bring a season-changing from across the state,
starting Sunday morning across the NW Parks, before briefly
stalling. The main push then sweeps through Sunday evening and
night, bringing much colder temperatures. The front will bring a
good chance for precipitation to the region, bringing rainfall to
most areas. Midlevel temps still show good cold air moving through
Sunday night, so some precip will be snowfall for the higher
elevations of the western mountains and the Parks. Some
accumulation is possible for the highest peaks, but that will
still be limited by amount of time of precip and somewhat warm
grounds still. The cold pool then settles across WY, bringing the
coldest night in a long time, with most areas near or below
freezing. The trough sweeps east quickly Monday, with ridging
taking back over for midweek. Temperatures recover quickly,
reaching back into the 60s and 70s by Wednesday. A weak shortwave
looking a bit more likely from the north on Thursday will bring
temps down just a bit, but will otherwise be a dry and tranquil
.AVIATION...(For the 00Z TAFS through 00Z Sunday)
Issued at 519 PM MDT Fri Sep 17 2021
VFR conditions look to continue throughout the TAF period. Gusty
winds at KBPI, KCPR and KRKS should diminish between 01Z-02Z. Gusty
winds look to be around again tomorrow starting between 17Z-19Z for
several locations. KCPR may see gusty winds as early as 14Z.
Please see the Aviation Weather Center and/or CWSU ZDV and ZLC for
the latest information on smoke, icing, and turbulence forecasts.
Issued AT 205 PM MDT Fri Sep 17 2021
Dry south-southwest flow with mostly clear skies continues the red
flag warnings over much of western and central Wyoming today. Some
areas are marginal with elevated conditions due to uncertainty of
winds, such as parts of the Wind River and Bighorn Basins. Higher
elevations will see RH values drop below 20 percent along with gusty
winds. Dry air remains in the higher elevations overnight as the
winds decrease for poor humidity recovery. Clouds and mid-level
moisture will increase over southwest WY Saturday morning,
bringing a slight chance of showers to far SW WY by noon Saturday.
As the moisture moves to the northeast during the day, showers
and thunderstorms are expected over much of western WY, especially
from the Wyoming Range toward the Wind Rivers. Some areas may see
more than 0.25" of precipitation by Saturday evening. Latest
models are showing areas of showers moving across Sweetwater
County during the early evening hours. This activity should keep
RH values above critical, thus moving the drier areas to the
northeast. Critical fire weather conditions then are only expected
from Natrona County into Johnson County Saturday afternoon, with
elevated conditions around the Bighorn Basin. Showers will linger
through midnight across central WY. Winds will remain breezy
Saturday night into Sunday morning as a strong storm system
approaches from the west-northwest. Expect a strong cold front
moving northwest to southeast Sunday afternoon through late
Sunday, bringing more showers and some mountain snow into Monday.
Red Flag Warning until 8 PM MDT this evening for WYZ277>279-285-
Red Flag Warning until 8 PM MDT Saturday for WYZ280-281.
Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
700 PM MDT Fri Sep 17 2021
Issued at 651 PM MDT Fri Sep 17 2021
Forecast remains mostly on track. Did add areas smoke in the
short term as a wildfire near Crawford NE has reduced visibility
in southwestern SD and the Black Hills region. HRRR near surface
smoke model keeps some smoke in the region at least through
Saturday evening. May have to reassess for Sunday as southwesterly
flow continues until around Sunday evening.
.DISCUSSION...(This Evening Through Friday)
Issued at 144 PM MDT Fri Sep 17 2021
Latest analysis shows a low amplitude upper ridge building over
the region. At the surface, a tight pressure gradient between high
pressure to the southeast of the forecast area and a strengthening
lee side trough over Montana is producing gusty southerly winds
across northeastern Wyoming and western South Dakota.
The winds will diminish a bit this evening, however areas of
breezy southerly winds will continue overnight. Low temperatures
tonight will be relatively mild, mainly in the mid-40s to lower 50s.
With high temperatures 10-20F warmer than average on Saturday,
gusty west to southwest winds are expected due to very deep
boundary layer mixing, especially over northeastern Wyoming on
Saturday afternoon. The southwesterly downslope component to the
winds will also lead to very low relative humidity over
northeastern Wyoming and much of western South Dakota on Saturday
For Sunday night into Tuesday, the upper trough will cross the
northern plains. The relatively quicker movement of the system
will lead to less moisture return and precipitation over the
forecast area than previously indicated. A sharp cold front will
cross the area on Sunday night followed by potentially
strong/gusty northwest winds. Thermal profiles suggest a chance of
snow possible in the higher Black Hills with little to no
The second half of next week will feature seasonable, typical
mostly dry Fall weather.
.AVIATION...(For the 00Z TAFS Through 00Z Saturday Evening)
Issued At 534 PM MDT Fri Sep 17 2021
Smoke from a wildfire in Nebraska will reduce visibility in areas
around KRAP this evening and Saturday, though visibilities should
stay above MVFR. VFR conditions are expected through the period.
Issued at 144 PM MDT Fri Sep 17 2021
Gusty southerly winds and minimum afternoon relative humidities
in the upper teens to mid 20s will lead to elevated fire weather
conditions. However, the relative humidity will drop around 15
percent over the western half of fire weather zones 314 and 315 in
Campbell County Wyoming, leading to critical fire weather
conditions in that area during the rest of this afternoon into
On Saturday, gusty west to southwest winds, as well as very dry
air, will create critical fire weather conditions over
northeastern Wyoming and much of western South Dakota. Poor
relative humidity recovery is also expected on Saturday night.
Elevated fire weather conditions are possible on Sunday ahead of a
cold front for Sunday night. This cold front will be followed by
gusty northwest winds as well as a chance of showers.
SD...Red Flag Warning from 10 AM to 9 PM MDT Saturday for SDZ319>333.
WY...Red Flag Warning from 10 AM to 9 PM MDT Saturday for WYZ314>318.
Red Flag Warning until 8 PM MDT this evening for WYZ314-315.