Forecast Discussions mentioning any of
"ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 12/20/21
Area Forecast Discussion
National Weather Service Louisville KY
940 PM EST Sun Dec 19 2021
.Forecast Update...
Issued at 940 PM EST Sun Dec 19 2021
A hole has opened up in the overcast from eastern Indiana to the
northern Blue Grass. This clearing trend is expected to continue,
and the sky forecast has been adjusted accordingly. With dew points
in the mid and upper 20s and light winds, combined with the clearing
skies, dropped tonight`s mins a bit in the valleys. Some patchy
valley fog around sunrise can`t be ruled out, if we do lose the low
clouds as expected.
&&
.Short Term...(This evening through Monday)
Issued at 311 PM EST Sun Dec 19 2021
A shortwave is over the region with a northeast to southwest
oriented axis of high pressure to the west. North to northeast
surface winds will remain over the CWA until the center of the high,
to our north, passes to our east causing a veering of winds towards
the south late tonight into the first half of tomorrow. This along
with area cloud cover has limited temperatures to the low 40s, and
low temperatures tonight will be dependent on if cloud cover remains
or dissipates. Guidance is varied on when the clearing could take
place. Satellite imagery shows clearing is beginning to take place
from west to east. This will likely be a slow clearing, but believe
skies will mostly clear tonight and allow temperatures to drop into
the mid to upper 20s.
Tomorrow, surface high pressure remains as weak WAA lifts
temperatures slightly warmer than today. Confidence remains low
regarding low level cloud coverage. Scattered low clouds could
remain over parts of the region before upper clouds work their way
northward into central Kentucky later in the day. If clouds are
able to completely clear, highs could be a few degrees warmer.
.Long Term...(Monday night through Sunday)
Issued at 222 PM EST Sun Dec 19 2021
Upper level pattern will start off quasi-zonal across much of the
lower Ohio Valley, resulting in quiet and dry weather as
northern/southern stream systems pass around us. A weak, dry front
will pass through the area Wednesday and drop temperatures to near
climatological normals, but upper level ridging will begin to
quickly build in from the west and result in a warming trend going
into the end of the week.
Highs by Friday could be in the low to mid 60s for many locations, a
good 15 degrees above climatological normals. It`s around this time
that we begin to see a trough and surface cold front swing into the
Plains and move toward us. Depending on the timing and path this
trough takes across the central U.S., we could see some showers
develop ahead of it. For now, will advertise low-chance PoPs on
Christmas Eve.
From Christmas Day and beyond, there remains a high degree of
uncertainty in the forecast. GFS/ECMWF ensembles vary considerably
on how some of the small scale features involving the trough across
the central U.S. evolves, which will have impacts on the local
forecast. For example, high temperatures for Christmas Day from
individual ECMWF/GFS ensemble members vary 30+ degrees for sites
across southern IN and central Kentucky, though the vast majority of
solutions are on the warmer side of climatology. Forecast for this
period reflects mainly a combination of ensemble means and NBM
output.
&&
.Aviation...(00Z TAF Issuance)
Issued at 636 PM EST Sun Dec 19 2021
Impacts: MVFR CIGs
Discussion: A large field of overcast ceilings just below the
MVFR/VFR threshold covers the area from the lower Great Lakes to
Upper Ohio Valley at TAF time. The future of these clouds is the
challenge for this forecast period. Latest SDF AMDAR soundings show
a very sharp inversion around 3400` and the cloud layer is very
shallow. So shallow that holes have managed to open up over IND and
CVG in the past hour...though the CVG hole is struggling.
Flow in the cloud bearing layer this evening is coming in form the
northeast off of high pressure over Ohio. As that high moves off to
the east tonight, flow just off the surface turns to the southeast
and possibly south by morning. As this happens the cloud deck is
expected to break up, and this is also seen in widening low level
dew point depressions on local sounding progs. So, will hold on to
the ceilings for several more hours, especially at SDF/LEX/HNB, but
then let it go.
Confidence: Low on timing of MVFR conditions.
&&
.LMK WATCHES/WARNINGS/ADVISORIES...
IN...None.
KY...None.
&&
$$
Update...13
Short Term...KDW
Long Term...DM
Aviation...13
Area Forecast Discussion...CORRECTED
National Weather Service Phoenix AZ
456 PM MST Sun Dec 19 2021
.UPDATED AVIATION DISCUSSION...
&&
.SYNOPSIS...
High pressure over the region will lead to a slow warming trend
early this week and temperatures returning to slightly above
normal by Monday. Cloudy skies will become a common feature as
early as Monday night, likely lasting through much of the week as
moisture feeds into the region out ahead of a very slow moving
Pacific weather system off the West Coast. Chances for rain are
then expected during the latter half of the week as the system
nears the region, with the best chances likely falling on Thursday.
&&
.DISCUSSION...
It is high and dry across the Desert Southwest this afternoon as
high pressure and subsidence spread across the region. Phoenix
aircraft sounding evolutions over the past 24 hours show notable
warming of the thermodynamic profile, which should yield surface
temperatures this afternoon a few degrees warmer than yesterday,
but still mostly in the mid to upper 60s across the lower deserts.
Phoenix has roughly a 30% chance of reaching 70 this afternoon,
based on the 13Z NBM. Lows tonight will remain seasonably chilly
with the dry conditions and clear skies. Ensemble forecasts
strongly support ridging persisting over the area through at least
the first half of the week with a small increase in pressure
level heights from today. This will result in another few degrees
of warming, with highs across the lower deserts climbing to around
70 and lows coming up several degrees. Increasing high clouds from
the southwest beginning Monday afternoon could limit full mixing
and warming potential in the afternoons, while also helping
insulate temperatures overnight.
The increasing high clouds Monday through Wednesday will be a
precursor to a significant moisture flux into the region heading
into Thursday. A strong low is currently circulating several
hundred miles off the western U.S. coast. This low, and the
overall trough pattern over the eastern Pacific, will help drive
deep southwesterly flow into the Desert Southwest. Forecast
soundings show moisture will mostly be elevated above 500 mb
from late-Monday through most of Wednesday before the more
significant and deeper slug of moisture advects into the region.
GEFS and EPS ensembles show a weak to moderate AR pushing into the
region, beginning as early as Wednesday night and persisting
through Thursday, with IVT values up into the 250-750 kg/ms range.
There are even some members showing above 750, or categorically a
strong AR. Compared to climatology, this forecast is up near the
99th percentile.
So moisture is expected to be quite high by later this week, but
there still remains some uncertainty in the evolution of the
trough over the eastern Pacific and any embedded shortwaves within
the flow. This is leading to uncertainty in overall rain chances,
specific timing of greatest rain potential, and QPF amounts. Majority
of ensemble members and ensemble means do seem to keep most of the
strongest dynamic forcing with the trough to the northwest of the
forecast area, which would virtually write off any chances for
thunderstorms and submit the region mostly to isentropic and
orographic forcing. The latest NBM shows quite a substantial
spread among the interquartile range (25th-75th percentile) of
24-hr QPF ending 5 AM MST Friday, from no measurable rain at all
in the CWA to 0.5" in Phoenix with higher amounts in the mountains
to the north and east. However, the chance is there for a
significant winter rainfall event with a roughly 10% chance of
1.0" or higher in Phoenix and into the mountains. These details
will and the spreads will hopefully be worked out and minimized
over the next few forecast iterations.
.PREVIOUS DISCUSSION...
Rain chances later this week are still very uncertain as model
ensembles are having a difficult time resolving the evolution of
the parent upper level low and any shortwaves the peel off of the
low. There remains a good number of members (~30%) that actually
have the low retrograding away from the coast and if this were to
happen, our rain chances will be fairly minimal. However, a slight
majority of members still show the main low drifting a bit
southward and then eastward late this week which would provide the
necessary dynamics for more widespread rainfall. NBM forecast
PoPs and model/WPC QPF are mostly unchanged from the past few
model runs, which call for the best chances (30-50%) falling on
Thursday. However due to the model uncertainty, rain chances are
spread out over several days, any time between late Wednesday
through Friday. The probabilities for impactful rainfall which
could cause localized flooding are still fairly low, but have
improved slightly over yesterday. The current NBM 24 hr
probability of exceeding 1" (Thu 12Z-Fri 12Z) is between 10-25%
over the Phoenix area to as high as 40% within the Tonto NF. There
are even some pockets of 10-25% probabilities of > 2" over the
Arizona high terrain. These higher rainfall possibilities are not
surprising given the expected deep sub-tropical moisture forecast
to move into the region during the middle part of the week, but
the question of available storm dynamics remains uncertain.
After the first likely shortwave affecting the region late this
week, broad troughing should still prevail over the Western U.S.
through next weekend. This may lead to another round of precip
potential, likely involving a colder air mass and lower snow
levels, unlike the very high snow levels (>9000 ft.) associated
with the Thursday/Friday wave. For now, the NBM is showing better
chances of this second system coming through next Sunday, but
being a week away, things can definitely change.
&&
.AVIATION...Updated at 2355Z.
South-Central Arizona including KPHX, KIWA, KSDL and KDVT:
High pressure to our north will force just enough of a gradient
flow to limit westerly diurnal switch at our metro area sites this
evening and tomorrow evening. Instead, a period of light/variable
winds can be expected before winds return to more steady E
overnight through much of Monday. An approaching disturbance well
to the west will begin to throw some high-level moisture at us by
late tomorrow. Expect FEW/SCT clouds above 25KFT after 21Z, going
BKN (still >20KFT) near or just beyond the end of this TAF
period. Bottom line, no TAF concerns this period.
Southeast California/Southwest Arizona including KIPL and KBLH:
No TAF concerns the next 24 hours. Winds will favor a north or
northwesterly component with speeds remaining below 8 kts. Mostly
clear skies will persist through midday tomorrow. After that time,
high clouds will spill in from the southwest well ahead of our
next disturbance. SCT/BKN clouds at around 25KFT are possible by
18Z. These clouds may lower to around 20KFT by just after 21/00Z.
&&
.FIRE WEATHER...
Tuesday through Saturday:
High pressure will continue through Wednesday, but with moisture
dramatically increasing across the region ahead of a nearly
stationary Pacific weather system off the West Coast. Wetting
rains will become possible as early as Wednesday, but more likely
Thursday and Friday, particularly across the Arizona high terrain.
Slightly above normal temperatures are expected through at least
Friday with a slight cooling trend possible into next weekend.
Min RHs through Wednesday will mostly range between 20-30% before
increasing to between 35-50% starting Thursday. Winds are likely
to remain fairly light through much period, but with some
breeziness possible Friday into Saturday.
&&
.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...None.
CA...None.
&&
$$
DISCUSSION...Benedict
PREVIOUS DISCUSSION...Kuhlman
AVIATION...Heil
FIRE WEATHER...Kuhlman