Forecast Discussions mentioning any of
"HRRR" "RAP" "RUC13" "RUC" "RR" received at GSD on 12/07/24
Area Forecast Discussion
National Weather Service La Crosse WI
556 PM CST Fri Dec 6 2024
.KEY MESSAGES...
- Above normal temperatures through Monday. Brisk winds
Saturday. Precipitation chances still remain low with the
weak Saturday frontal passage and the Sunday night through
Monday system.
- Colder with periodic snow showers/flurries Monday night
through Wednesday.
&&
.DISCUSSION...
Issued at 225 PM CST Fri Dec 6 2024
Tonight-Saturday: Above Normal Temperatures, Brisk Winds, Light
Snow Chance?
Heading into the weekend, the northwest flow that we have been stuck
in, finally breaks free and a more zonal flow takes shape. This will
allow for brisk southwesterly winds and resultant warm air
advection to move into the area. With this is mind, high
temperatures on Saturday will be in the low to mid 40s south of
I-94.
A subtle shortwave will move through the area on Saturday morning
will bring lower level clouds to the area. This wave is a part of a
bigger shortwave that will impact the Great Lakes and bring snow and
wintry mix to that area. The greatest lift remains north of the
forecast area. Another factor in play is how moist the column gets.
Between the different model soundings, there continues to be some
discrepancies how if the levels below 850 hPa reach saturation. The
HRRR and RAP support this solution while the NAM only has saturation
around 850 hPa with a dry sub-850 hPa layer. With that all being
said, there remains a low chance (~10%) for some flurries to occur
in the morning, mostly for Taylor/Clark counties.
Above normal temperatures continue through Monday.
Precipitation chances still remain low with the weak Saturday
frontal passage and the Sunday night through Monday system:
We remain in northwest flow aloft Saturday night as an area of
surface low pressure tracks through the U.P. of Michigan. This
weather system will drag a weak cold front into the area with the
850mb thermal ridge pushing south of the forecast area. At this time
if there is any light precipitation temperatures are above freezing
in the early evening, thus would not be impactful. Meanwhile...a
deepening area of low pressure drops southward into the Northern
Plains and the closed low over the Southwest U.S. lifts northeast
into the Mid-Mississippi Valley through Sunday. We continue to see
the swath of organized precipitation track across the U.P. of
Michigan/Great Lakes Saturday. Sunday afternoon through Monday,
we see the precipitation shields from the southern wave track
from the Mid-Mississippi Valley eastward toward the Tennessee
and Ohio River Valley and the northern system affect northern
Minnesota, the U.P. of Michigan and the Great Lakes. In-between
in the local forecast area, we don`t have much precipitation
forecast. The 06.00Z LREF has a 0 to 45% probability for
measurable precipitation with the 0-5% for northeast Iowa into
southeast Minnesota to 45% at Medford. The storm system
deepening over the Plains is just moving onshore Saturday, thus
should have better sampling for the models. Cluster analysis
shows some general agreement in the clusters for Sunday evening,
however slide difference remain in the extent of troughing
across the Northern Plains. We will continue to monitor the
forcing and relative humidity profile trends for light rain or a
nuisance wintry mix.
Colder with periodic snow showers/flurries Monday night through
Wednesday. Brisk winds with the cold front Monday night:
Although the closed 500mb low tracks northeast into Ontario Monday
night, shortwave trough energy will rotate through on the backside
of the low with a broad 500mb trough being carved out across much of
the central U.S. A much colder airmass builds in for Tuesday with
reinforcing cold air Wednesday. Highs are only forecast to be
in the teens and 20s. With temperatures aloft of -15 to
minus 20 it will be easy to get flurries or snow showers where
the moisture supports them. We`ll continue to monitor where the
center of the cold air tracks as strength and timing of this and
rh fields will affect how cold it will get and where/when the
snow showers/flurries will occur.
There are some differences in the EC/GEFS ensembles related to
how strong the winds will be with the front Monday. About 20%
of the EC members have wind gusts over 35mph at LSE while the
GFS only one out of 30 members. Temperatures remain in the 20s
for Thursday, but moderate for Friday.
&&
.AVIATION /00Z TAFS THROUGH 00Z SATURDAY/...
Issued at 555 PM CST Fri Dec 6 2024
The current MVFR stratus deck departs from west to east
overnight and retreats to north-central Wisconsin by sunrise.
VFR conditions look to be present for the daytime hours. Light
southwesterly winds overnight increase during the daytime hours
with gusts over 20-25 kts at times. Depending at the strength
of the morning inversion, there may be a brief period of LLWS
before deeper mixing takes place.
&&
.ARX WATCHES/WARNINGS/ADVISORIES...
WI...None.
MN...None.
IA...None.
&&
$$
DISCUSSION...Cecava/Zapotocny
AVIATION...Skow
Area Forecast Discussion
National Weather Service Missoula MT
245 PM MST Fri Dec 6 2024
.DISCUSSION...
KEY MESSAGES:
- Wintry mix Friday night through Saturday evening, then a
Pacific cold front will bring breezy winds and moderate snow
showers Saturday evening through Monday.
- The public should be VERY careful when traveling, especially in
"shady" areas on Saturday where black ice could catch you off
guard! Air temperatures could rise above freezing, but black
ice will still be a threat.
The cold pools are strong in many of the valleys. There continues
to be reports of freezing fog with some ice accretion and hoar
frost covering surfaces, and low clouds with spotty light snow has
also fallen in places.
A light southerly wind gradient develops today, mainly felt in the
higher terrain. This could help improve the fog and low clouds in
some locations, but the warming connected with these higher level
winds will cause the temperature inversions to strengthen even
more tonight through early Saturday. This will keep a chance for
freezing fog with light mixed precipitation going during this
time period.
The subtropical plume of moisture will be the driving force for
widespread mixed precipitation on Saturday. To place this moisture
in historical context, the amount of moisture contained ranges
from 200 to 300 percent of normal, or over 3 standard deviations
from normal. Looking at upstream upper air soundings, Vernon
British Columbia this morning had 37 degrees at 5600 feet MSL.
Just to the south Kelowna at an elevation of 1400 feet reported
freezing rain. We couldn`t rule out the precipitation starting out
as sleet or snow by daybreak across northwest Montana, but it will
quickly transition to freezing rain. The newer model guidance has
depicted a slower timing with the plume, but the latest HRRR shows
it centered over Grangeville to Missoula to Seeley Lake by 8 pm
MST / 7 pm PST Saturday evening. With the timing of this rain
occurring after dark, black ice formation is a real concern due to
how cold the ground temperatures are, especially in shady spots.
We had the highest confidence that ice accretion (accumulation)
would have the highest impacts across Lincoln County in northwest
Montana so that is where we issued a winter storm warning. This
event has some similarities to what occurred on December 12, 2018,
but one difference is the cold pools are a lot stronger with this
event.
With the strong westerly winds behind the front, gusty winds over
40 mph with snow showers are possible towards the Divide, i.e.
MacDonald Pass and Avon areas later Saturday evening. Transient
snow bands are possible, lingering into the overnight hours. With
these convective showers, some locations could pick up an inch or
two. Right now the high-res models depict several snow bands east
of Missoula, but as seen in the Dec 12, 2018 event, there was more
convective snow showers found across west-central Montana.
A secondary system will bring another round of wintry
precipitation, mostly in the form of snow during the day Sunday
through Monday. Low elevation locations in north-central Idaho may
experience either mixed precipitation or snow melting on the roads
which could become more an issue after sundown Sunday. There could
be additional winter weather advisories needed across the region,
including north-central and Lemhi County Idaho. Skiers will
appreciate the 6 to 12 inches of snow that could fall in the
mountains with this system.
A high amplitude ridge (meaning it will extend from California all
the way north into western Canada) will develop and bring more
stable conditions Tuesday and Wednesday. There will be a chance
that fog and low clouds could form.
Around 70 percent of the ensembles depict a cold trough passage
by next Thursday or Friday. This could bring another round of
wintry precipitation to the region.
&&
.AVIATION...Dense freezing fog and low stratus is expected to
remain in place across many valley locations across the Northern
Rockies through Saturday. An increased southerly wind aloft may
help to keep the chance for fog and stratus down for KHRF, but
with the cold air sloshing back and forth in the valleys, they
could experience all-of-a-sudden IFR or LIFR conditions. KMSO and
KGPI should keep the fog through Saturday and due to the strong
cold pools in place may not be able to improve that much. A plume
of anomalously high moisture with freezing levels up to 6000 feet
will bring freezing rain to the the region Saturday afternoon and
evening.
&&
.MSO WATCHES/WARNINGS/ADVISORIES...
MT...Winter Weather Advisory from 6 PM Saturday to 8 AM MST Sunday
for Bitterroot/Sapphire Mountains...Butte/Blackfoot
Region...Missoula/Bitterroot Valleys...Potomac/Seeley Lake
Region.
Freezing Fog Advisory until 2 PM MST this afternoon for
Missoula/Bitterroot Valleys.
Winter Weather Advisory from 2 AM to 10 AM MST Saturday for
Kootenai/Cabinet Region.
Winter Storm Warning from 10 AM to 11 PM MST Saturday for
Kootenai/Cabinet Region.
Winter Weather Advisory from 2 AM Saturday to 8 AM MST Sunday
for Flathead/Mission Valleys...Lower Clark Fork Region...
West Glacier Region.
ID...Winter Weather Advisory from 5 PM Saturday to 7 AM PST Sunday
for Lower Hells Canyon/Salmon River Region...Northern
Clearwater Mountains...Orofino/Grangeville Region...
Southern Clearwater Mountains.
Freezing Fog Advisory until 1 PM PST this afternoon for
Orofino/Grangeville Region.
&&
$$