Forecast Discussions mentioning any of
"HRRR" "RAP" "RUC13" "RUC" "RR" received at GSD on 12/19/20
Area Forecast Discussion
National Weather Service La Crosse WI
527 PM CST Fri Dec 18 2020
.SHORT TERM...(This evening through Saturday)
Issued at 245 PM CST Fri Dec 18 2020
Forecast concerns in the short term will be precipitation type,
coverage, and snow amounts with the light wintry mix of
precipitation that moves through tonight.
Latest radar observations and metars since 12Z show a leading area
of echoes aloft with some light precipitation in northeast South
Dakota and also farther to the north across northern MN through late
morning. Early this afternoon, the snow band has increased from
Hibbing toward Alexandria with moderate snow. The 12Z MPX
sounding had 0.29 inches of precipitable water yesterday at 12Z
and was up to 0.40 inches this morning. A narrow moist layer
around 1200ft was seen with a deep dry layer to around 11,000ft.
Visible satellite imagery has some thinning and also a little
convective look to some of the development over parts of southern
MN, northern IA, and northeast Neb. The lowest visibilities with
the snow are across northern Minnesota with 1/4 mi vsby.
Frontogenesis is stronger across northern and central MN with steep
lapse rates of 6.5 to 7.5 deg C. The left front exit region of the
jet is increasing upper level divergence across Iowa into the rest
of the forecast area by 00Z then shifts off to the east. The CAMs
show and uptick in precipitation as the trough/upward motion
increase this evening. This can also be seen in the 0-2km RAP rh as
it fills in 23Z and beyond. The RAP forecast soundings early this
evening do have ice being introduced with lift in the dendritic
growth zone. Meanwhile...the surface front is pushing into western
MN and eastern Nebraska. Will maintain higher pops, however light
precipitation amounts. Due to the warmer surface temperatures a
mix is possible, but once the stronger lift occurs have it more
snow. Snow amounts mostly a trace to a half an inch.
Temperatures drop from the upper 20s to the teens behind the front.
This will need to be watched as we are starting above freezing. Some
locations will see their Saturday high just after midnight with the
colder air coming in for Saturday.
.LONG TERM...(Saturday night through Friday)
Issued at 245 PM CST Fri Dec 18 2020
At least a few snow chances through the extended with
temperatures turning much colder for Christmas Eve/Christmas Day.
The shot of cool air behind the front Saturday is short-lived with
return flow already by Saturday night. Upstream, the next wind
shift is pushing through the Dakotas and comes through dry Sunday.
The next clipper-type system is a little bit more robust and dives
southward toward the forecast area Sunday night into Monday. The
GFS/EC/NAM/CMC all have a light snow signal, however our blends are
dry. Did go ahead and keep the chance for light snow going. Mostly
less than an inch, however some of the EPS ensembles for Medford
have some higher 1 to 3 inch amounts.
Ridging for Tuesday, then the active pattern continues with a
Central Plains trough. The strength of the trough is in question
and this can be seen with the EPS. Again...as the ensembles are
showing a trace to 1 inch, however there is a large spread with a
few in the 3 to 8 inch range. The very cold temperatures associated
with the surge of cold air Wednesday night if combined with moisture
would be easier to snow. Temperatures plunge Wednesday night and
remain cold Christmas Eve/Christmas Day with sub-zero wind chills.
&&
.AVIATION...(For the 00Z TAFS through 00Z Saturday evening)
Issued at 526 PM CST Fri Dec 18 2020
A trough of low pressure will move through the area tonight
bringing with it a mix of light rain and snow. Expecting this in
the 02-06z time frame at KRST and 04-08z time frame at KLSE. Very
minor accumulations of snow are expected with a trace to perhaps a
1/4. Look for vsby generally in the 5-6sm range in the
precipitation, but could see some further reductions if we see a
more vigorous snow shower pass through. Otherwise, deteriorating
cigs expected with the precipitation with IFR lingering into
Saturday morning. Gradual improvements Saturday with KRST VFR by
19z while KLSE remains in MVFR through the afternoon. Plan on
light south winds this evening, swinging to the northwest in the
9-15kt range after midnight into Saturday.
&&
.ARX WATCHES/WARNINGS/ADVISORIES...
WI...None.
MN...None.
IA...None.
&&
$$
SHORT TERM...Zapotocny
LONG TERM...Zapotocny
AVIATION...DAS
Area Forecast Discussion
National Weather Service Charleston SC
1008 PM EST Fri Dec 18 2020
.SYNOPSIS...
High pressure will prevail through Saturday. Complex low
pressure will impact the region Saturday night into Monday.
Then, high pressure will build into the area through mid week.
A strong cold front is expected to cross affect the area
Thursday, followed by colder high pressure late week.
&&
.NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
There are a few subtle nuances in the forecast tonight, with the
main concern regarding how much of the stratocumulus over the
Atlantic advects onshore, and whether or not any fog/freezing
fog can occur.
Satellite images have shown a distinct trend toward increasing
stratocumulus brushing onshore of coastal McIntosh and coast
Liberty County. This is the result of a slightly more favorable
onshore trajectory than further north, along with increasing
isentropic ascent. As a result we have increased cloud cover
those areas through the night, which will impact the diurnal
temp curve. Elsewhere it`ll stay clear or mostly clear with just
some wisps of thin cirrus in places.
As a cold as it`ll get, we anticipate that steam fog will form
over and near warmer bodies of water inland such as ponds,
creeks and marshy regions. There will also be at least some
ground fog, especially across Georgia. Given the sub-freezing
temps this would be considered freezing fog, albeit shallow in
nature.
The NAM and RAP indicate that fog could become more than just
ground fog, but condensation pressure deficits for the moment do
not look low enough for such an occurrence.
Expect a huge range of min temps, from as cold as the upper 20s
northwest tier to around 40F near the coast where north-
northeast breezes and/or clouds will modify the otherwise cold
air mass.
Previous discussion...
Other than thin cirrus at times through the night, skies will
remain clear as the marine-induced stratocumulus stay generally
offshore. Toward morning some of them could advect onshore of
McIntosh County as the weak isentropic ascent develops and the
low level flow veers to the east-northeast. Strong radiational
cooling is already underway and will allow for freezing temps to
cover inland locations all the way to near the US-17 corridor.
Coldest will be the northwest tier where upper 20s will be
common. It you`re at or near the coast, min temps will average
35-40 degrees.
&&
.SHORT TERM /6 AM SATURDAY MORNING THROUGH MONDAY/...
Saturday: High pressure will prevail at the surface and aloft. After
a cold start, temperatures will only recover into the middle to
upper 50s Saturday afternoon, a few degrees below-normal for mid-
December. Also, ahead of low pressure developing upstream, high
clouds will spread across the region, especially during the
afternoon.
Saturday night: An upper trough to the west will amplify and advance
toward the east. Deepening moisture transported from the southwest,
an overrunning regime supported by an inland surface wedge of high
pressure/a coastal trough and increasing deep layered forcing for
ascent should support increasing coverage of showers, especially
after midnight. Pops later Saturday night range from around 50
percent inland to around 30 percent far south/east. Higher PoPs
could eventually be required for inland/northern locales. Low
temperatures should range from 40-45F across a wide swath of inland
counties to 45-50F on the beaches.
Sunday: The initial episode of overrunning rain/showers will advance
toward the east/northeast during the morning. Morning PoPs range
from around 50 percent most areas except 30-40 percent far south,
and higher PoPs could eventually be required for some
northern/inland areas. Then, a relative lull in precipitation
coverage could develop between the morning event and another round
of forcing for ascent/enhanced moisture produced by the approaching
upper trough/associated surface low ejecting northeast from the Gulf
of Mexico later in the day or Sunday night. However, sufficient
uncertainty exists regarding the interval between these events, so
differences between Sunday morning/afternoon PoPs are subtle.
Otherwise, expect a mainly cloudy day with high temperatures ranging
from the lower/middle 50s inland to the upper 50s/lower 60s near the
coast and across southern counties.
Sunday night into early Monday: A sharp mid/upper level trough and
associated surface low should cross the region, producing another
round of rain/showers. Elevated instability could even support a few
lightning strikes/pockets of heavier rain, although the probability
for thunder remains too low to mention within forecasts. Per area
consensus, PoPS are capped around 50 percent, but locally greater
PoPs could eventually be needed. Low temperatures 45-50F should be
common.
Monday: Some deterministic models push drier air across the region
early, while other guidance depicts persistent moisture/showers
associated with the upper low/cold pool aloft. Thus, the decrease in
PoPs remains conservative for this forecast, with at least slight
chance PoPs hanging on along the coast into the afternoon. The
uncertainty regarding PoPs also translates the potential high
temperatures. For now, forecast highs remain close to normal, in the
upper 50s to lower 60s.
&&
.LONG TERM /MONDAY NIGHT THROUGH FRIDAY/...
Surface high pressure west of the area Monday night will build
eastward crossing the area Tuesday night and Wednesday before moving
into the western Atlantic by Wednesday night. Models are all in
agreement that a cold front will move through the area Thursday into
Thursday evening with some rain chances starting Wednesday night and
continuing perhaps into Friday. There is decent confidence in this
part of the extended forecast. However, there is a lot of
uncertainty regarding the strength of the cold front and how cold it
gets behind the cold front and this will impact temperatures on
Christmas Day and into the weekend.
&&
.AVIATION /03Z SATURDAY THROUGH WEDNESDAY/...
VFR through 00Z Sunday.
Extended Outlook...Increasing probability for MVFR/IFR ceilings from
late Saturday night through Sunday night/early Monday. VFR
conditions return Monday and persist through Wednesday.
&&
.MARINE...
Overnight: High pressure will dominate with primarily N-NE flow
over the waters. A lingering tight pressure gradient will make
for 10-15 kt winds with occasional gusts up to 20 kt. Seas will
generally range from 2-4 feet across the waters.
High pressure will produce NE winds 10-15 kt and seas 2-3 ft
Saturday into Saturday evening. As a coastal trough develops later
Saturday night into Sunday night, wind speed should remain below 15
knots, and wind direction will vary with time and will depend on the
position of the coastal trough. Then, as surface low pressure tracks
NE along the Gulf Stream, a tightening surface pressure gradient
will support strengthening offshore winds starting later Sunday
night, and offshore winds could support Small Craft Advisory
conditions in some areas, especially AMZ350 and AMZ374, Monday into
Tuesday. Then, more tranquil conditions should develop for a brief
period around midweek before a strong cold front advances through
the region Thursday.
&&
.CHS WATCHES/WARNINGS/ADVISORIES...
GA...None.
SC...None.
MARINE...None.
&&
$$
NEAR TERM...
SHORT TERM...SPR
LONG TERM...MTE
AVIATION...
MARINE...
Area Forecast Discussion
National Weather Service Lincoln IL
927 PM CST Fri Dec 18 2020
.SYNOPSIS...
Issued at 247 PM CST Fri Dec 18 2020
Mild temperatures with highs into the lower to mid 40s are
expected Saturday, but a cold front will bring a chance of light
rain or drizzle through much of the day. The weather will be dry
Sunday and temperatures will remain mild in the lower 40s for high
temps.
&&
.UPDATE...
Issued at 920 PM CST Fri Dec 18 2020
Expansive cloud cover will remain across our CWA overnight, due to
strong WAA southerly flow bringing increasing mid-level moisture.
VIRGA will work to progressively saturate the lower levels the
rest of the night. Radar returns already showing up across our
southern counties are deceptive due to a dry sub-cloud air mass.
Cloud heights of 6K FT are starting to develop as of 9 pm.
However, we do not expect precip reaching the ground until after
midnight, with better chances toward sunrise on Saturday.
Have continued the start of precip in the forecast at 11z/5am, but
a few sprinkles could reach the ground as early as 3 am for the
I-72 corridor where the high res guidance is pointing toward
saturation developing sooner there.
The cold front is projected to reach our western counties between
6-9am Sat morning, then progress slowly eastward to near the I-57
corridor by 00z/6pm. Rainfall in most of our CWA will be spotty
Saturday morning into early afternoon, before a more focused area
of rainfall develops ahead of the front for our southeast counties
Saturday afternoon. That progression is already handled well in
the forecast. Only minor updates were done to the short term
forecast, with minimal impact on the forecast details.
&&
.SHORT TERM...(This evening through Saturday)
ISSUED AT 247 PM CST Fri Dec 18 2020
Low pressure is centered over eastern NE this afternoon and is
progged to shift across southern WI Saturday with a trailing cold
front sweeping across Illinois. Meanwhile a southern stream
shortwave is over the Desert Southwest this afternoon and expected
to shift east across the lower Mississippi Valley Saturday. This
southern wave will keep deeper moisture well south of Illinois
which will result in fairly light and possibly sparse precip
coverage across central Illinois with frontal passage. Forecast
soundings show the column sufficiently warm for rain through the
event, and while the column does cool some behind the front, loss
of ice nuclei and above freezing surface temps will result in just
some drizzle. Finally, there is a fairly zonal pattern aloft with
little in the way of cold air behind the cold front. Temperatures
will warm into the lower to mid 40s ahead of the front Saturday,
and will remain seasonably mild behind the front with highs in the
lower 40s Sunday.
.LONG TERM...(Saturday night through Friday)
ISSUED AT 247 PM CST Fri Dec 18 2020
The upper pattern will become increasingly amplified through the
upcoming week with well above normal temperatures early in the
week giving way to an arctic air mass late in the week. On Monday,
a clipper system will bring a chance of precipitation to the
northern half of Illinois. There remains some uncertainty on the
exact track of this system and for now there is only a slight
chance of precip along and north of I-74. Meanwhile, upper ridge
will slowly build east through the first half of the week
supporting above normal temperatures with highs in the mid to
upper 40s while a few southern locations push into the lower 50s.
A strong shortwave will move onshore over the Pacific Northwest
early next week and carve out a deep trough as it moves across the
mid section of the country by Wednesday. A deep surface low will
develop over the northern Great Plains in response and shift east
into the Upper Great Lakes Wednesday with a strong cold front
pushing across Illinois. Temperatures will plunge behind the front
as h85 temps of around -20C advect over the Midwest through the
latter half of the week. This will translate to afternoon highs in
the 20s and overnight lows in the teens and single digits. Near
zero or below zero wind chill values appear possible for
Christmas eve and Christmas day.
&&
.AVIATION...(For the 00Z TAFS through 00Z Saturday evening)
Issued at 556 PM CST Fri Dec 18 2020
South-southeast wind gusts will diminish by early this evening,
but sustained winds will continue in the 10-15KT range. With the
decrease in surface winds and the increase of a low level jet
over the region, LLWS will become a concern to aviation. 50KT
winds will develop just above the ground and continue overnight.
A cold front will bring spotty rain on Saturday, mainly southeast
of the terminal sites. The latest HRRR and RAP updates show a
continued potential for IFR and drizzle during the day tomorrow.
Therefore, we did include a trend to IFR clouds late in the 00z
TAF period.
&&
.ILX WATCHES/WARNINGS/ADVISORIES...
NONE.
&&
$$
UPDATE...Shimon
SYNOPSIS...Deubelbeiss
SHORT TERM...Deubelbeiss
LONG TERM...Deubelbeiss
AVIATION...Shimon