Forecast Discussions mentioning any of "HRRR" "RAP" "RUC13" "RUC" "RR" received at GSD on 11/26/23

Area Forecast Discussion...Updated

National Weather Service Lincoln IL

 844 PM CST Sat Nov 25 2023

.SYNOPSIS...
Issued at 844 PM CST Sat Nov 25 2023

Our first measurable snow of this winter arrives later tonight into
Sunday morning, when a weather system brings light accumulating snow
to areas north of a line from Shelbyville to Danville. A Winter
Weather Advisory is in effect for counties along and west of the
Illinois River where snow amounts at or above 2 inches are most
probable.

&&

.UPDATE...
Issued at 844 PM CST Sat Nov 25 2023

Evening surface map indicated 1016 mb low pressure over northeast
OK. Aloft, a shortwave was noted on water vapor imagery over
southwest KS, with a good plume of mid level moisture ahead of
this feature expanding from the central Plains east into western
IA and MO. Radar returns already lighting up as far east as west
central IL, but this is all virga until much farther west across
central IA and northwest MO. 00z KILX sounding showed a sharp dry
layer below 850 mb which will take time to saturate, but as
stronger lift overspreads west central IL later tonight, snow will
rapidly develop for areas west of the IL River between midnight
and 3 am, then spread east toward I-55 through sunrise. Overall
thinking on timing and amounts has not changed much from the
previous forecast. Still seeing signs of transient stronger f-gen
banding from about 3-10 am northwest of I-55, which should produce
periods of moderate snow and amounts by mid to late morning of
1-3 inches in the advisory area. Slick travel is likely in this
area through around 8-10 am, then with surface temperatures rising
above freezing any accumulations on roadways should be ending.
Southeast of the advisory area, most of the snow will be falling
during the daylight hours with temperatures above freezing, so
minor accumulations on grassy and elevated surfaces are possible
into east central IL, with little to no issues anticipated on
roads.

25

&&

.DISCUSSION...
Issued at 330 PM CST Sat Nov 25 2023

Key Messages:

*  Our first measurable snow of this winter will arrive later
   tonight and continue through around midday on Sunday. Slippery
   roads are expected for holiday travelers returning home.

* Much cooler temperatures arrive early in the week with wind chills
  in the single digits expected on Monday and Tuesday mornings.

* Things turn drier going into the new week with temperatures
  regulating back to near normal by midweek. The next chance of
  precipitation comes Thursday night and Friday, mostly as rain.

High pressure extending across Illinois this afternoon will give way
to a storm system developing along the southern border of Colorado.
That low will eject across the southern Plains and phase with an
upper level trough digging SE from the Canadian Prairies.
While surface cyclogenesis appears to be relatively weak as winter
storms go, there are a couple of features that could trigger some
enhanced snowfall. There is a potential vorticity (PV) anomaly
associated with the Plains low that will advance eastward and
interact with the leading edge of a mid-level jet max across the
northwest half of Illinois. In concert with that, HRRR and other
high resolution guidance are pointing toward enhanced Fn Petterson
frontogenesis in the 850-750 layer aligned in SW to NE bands in that
same area. That could briefly boost snowfall rates over 1/2" per
hour. DESI HREF guidance indicates around 40-60% chance of greater
than 2" snowfall in bands west of I-55. Additionally, winds will
generally be light during the snowfall, so little fracturing of the
crystals is expected which can tend to reduce snowfall totals due to
compaction of the smaller flakes. So snowfall can accumulate more
efficiently under lighter wind conditions.

The 12z model suite shifted some higher QPF amounts farther east
toward I-55 after 6 am Sunday morning across our CWA. Which
increased snowfall amounts farther east as well. Therefore, after
collaboration with surrounding offices, we added a row of counties
on the east side of the Illinois River to the Winter Weather
Advisory. This is still a low end advisory, but since it the first
measurable snowfall of the season and it is occurring when a high
number of travels are expected after Thanksgiving, we defaulted to
issue the advisory to alert everyone to slippery conditions.

The snowfall will generally shift east of central Illinois by
early afternoon, but a trailing trough sweeping across northern
Illinois will trigger spotty light snow or flurries into Sunday
evening across areas along and north of I-74. No additional
accumulation will occur from that however.

Sunday night, high pressure will advance into the area from the
Plains, increasing the pressure gradient across the area. Those
winds will remain brisk through Monday morning when surface temps
dip into the upper teens to low 20s. Apparent temperatures will be
in the single digits Monday morning. Very similar conditions are
likely Tuesday morning as 850mb temps dip near -10C.

By mid-week, the high pressure will shift into the SE U.S.
providing our area with return flow and moderating temperatures.
By Friday, highs will climb above normal to near 50. However, rain
chances will return Thursday night and Friday.

Shimon

&&

.AVIATION...  (For the 00z TAFs through 00z Sunday Evening)
Issued at 533 PM CST Sat Nov 25 2023

The focus of the aviation forecast is accumulating snow that
spreads across the terminals late tonight through midday Sunday.
Latest guidance has trended more pessimistic on ceilings and
visibilities, and have adjusted TAFs accordingly to show IFR
conditions from around 10-20z. Conditions may drop to LIFR at
times during the peak of the snow from 12-16z, but did not include
this yet as confidence is low. As the system pulls away a gradual
improvement is expected during the afternoon with MVFR conditions
returning by late afternoon. Winds will be light from the east
overnight and south Sunday morning, then become gusty from the
west on the back side of the system.

25

&&

.ILX WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from midnight tonight to noon CST Sunday
for ILZ027>031-036-037-040-041-047.

&&

$$



Area Forecast Discussion

National Weather Service Pocatello ID

 153 PM MST Sat Nov 25 2023

.SHORT TERM...Tonight through Tue afternoon. Strong high pressure
leaves the period dry and clear aloft, with the only forecast
concerns being fog and stratus below the radiation inversion. If
it is deep and widespread, afternoon highs may not warm up much;
on the bright side, overnight lows will not be as cold. The fog
could produce black ice on road surfaces for morning travelers. Do
not believe a dense fog advisory will be necessary, but expect
whichever develops (fog or stratus) to be fairly widespread
tonight. Temperatures in the mountains, where there will be more
sunshine, should get above seasonal normals by Mon. For Snake
River plain locations, perhaps by Tue. Wind under high pressure
not expected to amount to much. Messick

.LONG TERM...Wednesday through Saturday.
The beginning of the long term forecast is a quiet one. Our ridge of
high pressure will break down a bit on Wednesday thanks to a low
passing to our south, but all of the associated moisture remains to
our south. Weak ridging over eastern Idaho on Thursday will keep
highs in the low to mid 30s for most, just a couple of degrees
cooler than highs on Wednesday. In fact, highs will stay in the 30s
for most of next week and weekend. We stay dry on Thursday, too, but
that could begin to change as early as Thursday night into Friday as
our next system begins to move in. The GFS drops a shortwave trough
out of British Columbia and through Idaho on Friday, bringing a
chance for snow showers with it before bringing yet another
shortwave through on Sunday with another round of showers. To no
one`s surprise, the ECMWF doesn`t paint quite the same pictures. It
drops another low to our south on Friday and keeps us dry, but does
bring a shortwave in on Saturday and then keeps us in northwesterly
flow aloft and brings in a few waves of snow showers from Saturday
all through the weekend. While all WPC clusters do show some type of
troughing impacting southern Idaho for Friday and Saturday, they all
vary in intensity and placement. We still have almost a week to work
out these wrinkles, but it does look like we`ll head back into a
more active pattern in early December.
AMM

&&

.AVIATION... 18Z Saturday through 18Z Sunday.
We remain in northerly flow aloft through the weekend, keeping us
cold. In addition to the cold, we keep around this low-level
moisture and very little wind, meaning freezing fog and low stratus
will be the name of the game for now. The bulk of the low stratus
was hanging out near BYI through the morning, but is working up the
Snake Plain, reaching PIH and towards IDA, and into surrounding
valleys like SUN. In fact, we`ve already seen IFR CIGs at SUN and
MVFR CIGs at PIH. While conditions are expected to improve around 3Z
at SUN with the usual diurnal wind shift, CIGs WILL stay low/drop
elsewhere. There is about a 30 to 50 percent chance of CIGs less
than 1000 ft overnight into early Sunday at BYI, IDA, and DIJ. At
PIH, it`s about a 20 percent chance, but still near a 50 percent
chance for at least MVFR CIGs. There is about a 30 to 40 percent
chance that low CIGs drop to LIFR levels around 06Z Sunday at IDA,
BYI, and DIJ. The HRRR also wants to bring these sites down to LIFR
levels and bounce PIH back and for between IFR/LIFR levels tonight
through early Sunday.
AMM

&&

.PIH WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$