Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 02/05/20

Area Forecast Discussion

National Weather Service Chicago/Romeoville, IL

 918 PM CST Tue Feb 4 2020

.UPDATE...
917 PM CST

Main focus continues to be on our accumulating snow event starting
mid to late afternoon Wednesday and gradually spreading northward.
Will not be issuing any winter headlines for the event with this
update due to lingering uncertainty, so will defer to the incoming
midnight shift for this. For tonight, we`re under the influence
of cool and dry northerly flow from high pressure ridging
extending from the northern Plains to northern Lake Michigan. The
forecast is in good shape, with most areas staying precipitation
free due to the aforementioned dry air feed. In fact, 00z DVN RAOB
and aircraft soundings out of MDW show exceptionally dry air from
just under 900 mb to just under 800 mb, with this dry layer
extending up to above 700 mb.

Lower and mid level frontogenetical circulations are responsible
for the radar echoes on regional mosaic, but much of this is virga
due to the very dry air at the lower and mid levels. Only areas
likely to see any falling precip are mainly along/south of US-24,
which would mostly be flurries, per obs just south at TIP and LAF
that have had occasional 9-10 mile visibility light snow this
evening. Otherwise, occasionally blustery conditions continue this
evening, where ORD still gusted to 30 kt on the 03z ob. These
persistent brisk northerly winds will keep the risk for lakeshore
flooding going through the night.

Turning ahead to the accumulating snow developing Wednesday PM,
the antecedent very dry air aloft described earlier is something
that we`ll have to account for, especially with northward extent.
It appears the initial surge into Wednesday PM could be driven
largely by lower and mid level frontogenesis, starting south of
I-80 and then gradually lifting northeastward. Frontogenesis
driven banding often leads to a sharp cut-off on the northern
side of the snow bands due to subsidence, plus we`ll have the dry
air with northern extent. The models do a good job of indicating
the potential for f-gen driven banding in a general sense, but
less so with the exact location of it, especially 18+ hours out,
which can still vary notably north/south from explicit QPF
placement on the model.

This all said, "threat" zone for the intial moderate to perhaps
brief heavy snow rates (could produce a period of 1"+/hr rates),
appears to be for areas south of I-88, and particularly I-80 and
south. This initial more banded snow would occur into the mid
evening, followed by a broader area of light to moderate snow in
the system deformation the rest of the night and into Thursday.
Even this portion of storm could have a fairly sharp cut-off
somewhere over northwest 1/3 of CWA. Into Thursday morning could
also include a lake enhancement component for portions of
northeast IL and northwest IN as lake induced thermodynamics
improve. All of these wildcards lead to lower than average
confidence this close to the event. For these reasons, will let
the midnight shift take a look at the complete 00z guidance suite
and observational trends with respect to which counties to
include in winter headlines (likely advisory).

Castro

&&

.SHORT TERM...
310 PM CST

Through Wednesday night...

The primary forecast challenges continue to center around snow
amounts Wednesday night and Thursday.

Surface high pressure over the Upper Midwest early this afternoon
will shift eastward over the western Great Lakes by Wednesday
morning. As it does so, northeastern winds in the lower-levels
will continue to allow a drier airmass to shift southward into
northern IL tonight. This drier airmass, which looks to be
centered around 850 mb, could impact how fast the snow is able to
develop into northern IL Wednesday evening.

Much of the 12z model guidance has backed off some on QPF amounts
over much of the area north of I-80 in IL, where this drier
airmass could take some time to saturate Wednesday evening. This
adds some uncertainty in the start times of the snow for areas
along and north of I-80. However, snow (possibly moderate to
heavy at times) is likely to develop during the afternoon and
evening to the south of I-80 over my central IL and northwest IN
counties. This band of snow will be driven by some stronger
mesoscale forced ascent associated with a strong band of 850-700
mb frontogenesis setting up along the northeastern quadrant of
the deepening 850 mb low shifting over the Mid- Mississippi Valley
into Wednesday evening. Given that this frontogenetic circulation
also looks to coincide with the area of better upper level jet
dynamics, we could see some moderate to heavy snow over my
southern counties late Wednesday afternoon and evening. This could
produce a quick 2 to 4 inches of snow over my south through
Wednesday night.

Precipitation type will need to be monitored, however, especially
over my far southeastern (including parts of Iroquois county IL
and Benton county IN) counties. The warm nose aloft could try to
shift northward into this area Wednesday evening ahead of the 850
mb low. While confidence on it making it into this area remains
low, if it does, some mixing potential of sleet and freezing rain
would be possible and this would lower snowfall amounts.

Otherwise, areas north of I-80 may not experience any light snow
potential until mid to late Wednesday evening and night. The snow
(mainly light) then looks to continue across much of northern IL
into Thursday. Some potential for lake effect/enhancement also
could come into play for parts of northeastern IL and northwestern
Indiana on Thursday into Thursday evening. I did not get to
focused on the lake effect potential during this period, but it
certainly could result in some higher accumulations near the
lake.

Overall, snowfall totals north of I-80 and inland from the lake
remain of lower confidence due to possibility of the start time
being delayed due to drier air and a lighter intensity into
Thursday. For this reason, we opted to hold off on issuing any
winter weather advisories at this time. If current trends hold
true, one will be needed for areas south of I-80, and potentially
parts of northeastern IL as well. Since the snow is not expected
to start in my south until mid to late Wednesday afternoon, we
will let the midnight shift get another look at the observational
trends.

KJB

&&

.LONG TERM...

Friday through Tuesday...

150 PM...There are several periods where some form...mainly
light...precipitation will be possible but confidence is fairly
low.

Its possible lake effect snow showers may still be lingering
Friday morning but should be quickly dissipating and shifting
further east as winds turn northwesterly. There appears to be a
weak wave moving across the area Friday night...to varying degrees
with the 12z runs. Enough of a signal however to include a chance
of flurries for this time period. There is another clipper like
system that swings across the western Great Lakes in the Sunday
time period...more pronounced on the gfs/gem. Have maintained low
chance pops for Sunday afternoon/Sunday night though the ecmwf is
generally dry for the cwa. Beyond this time period...quite a bit
of uncertainty into early/mid next week as the ecmwf has a more
developed storm system affect the area. Given how much the current
time period has changed from mid/late last week...made no changes
to previous forecast or new blended guidance and maintained low
chance pops for next Mon/Tues. cms

&&

.AVIATION...
For the 00Z TAFs...

Aviation forecast concerns:

-Gusty northeast winds slowly diminishing tonight.

-Patchy MVFR ceilings possible through period.

-Snow developing late Wednesday/Wednesday evening.

Late afternoon surface analysis indicates high pressure centered
across the upper Midwest, with dry anticyclonic low level flow
into the region. Northeast winds have been strong and gusty
through the day, thanks to the tight surface pressure gradient
between the high to our north, and a stalled frontal boundary
across the Ohio Valley, though the gradient is expected to ease
with time tonight as the high shifts east across the Great Lakes.
Gusts in the 25-30 kt range early this evening will settle back
into the 15-20 kt range by late evening. Mixed layer depth appears
to remain around 2000-2500 ft through Wednesday, which may
support some occasional gusts into the 15-20 kt range at times.
With the advection of drier low level air from the northeast, MVFR
cloud deck in the 2000-2500 ft agl range have largely scattered
out by late this afternoon, with VFR conditions currently across
the region. Persistent northeast flow, which will continue through
the period, may produce some occasional MVFR ceilings, though
confidence is fairly low on this being widespread or persistent.

By Wednesday afternoon, a mid-level disturbance will approach the
region from the southwest, and will track along the frontal zone
to our south late Wednesday through Thursday. Model guidance is in
general agreement in developing light snow into the GYY/MDW areas
by evening, with the northern edge of the precip shield slowly
lifting north toward ORD/DPA during the evening. Some uncertainty
as to exact details/timing of this, as well as how quickly MVFR or
potential IFR vsbys develop, though feel reasonably confident that
TAF thinking shows the trend well. IFR conditions and accumulating
snow are likely Wednesday night into Thursday, beyond the end of
the current TAF period.

Ratzer

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Lakeshore Flood Advisory...ILZ006 until midnight Wednesday.

     Lakeshore Flood Advisory...ILZ014 until 6 AM Wednesday.

IN...Lakeshore Flood Advisory...INZ001-INZ002 until 6 AM Wednesday.

LM...Small Craft Advisory...nearshore waters
     until 9 AM Wednesday.

&&

$$

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Area Forecast Discussion

National Weather Service Lubbock TX

 554 PM CST Tue Feb 4 2020

.AVIATION...
Not a particularly good night for flying in West Texas especially
for non-FIKI aircraft. We may see conditions temporarily come up
to VFR at KPVW early this evening, but it probably will not last.
A few snow showers have moved through the region and many reports
of rime icing have been received (SFC- 150kft AGL) via monitoring
ATC comms. AMDAR soundings show a below freezing profile all the
way up from KLBB north and west though a thin unfrozen layer is
indicated around 8500MSL to the ESE of KLBB, conditions are
expected to worsen toward midnight local as another batch of
wintry precipitation overspreads the area. Current indications
suggest a sharp gradient of heavy snow could form from about KLBB
to the southeast along a NE/SW line. The data suggest increasing
snow totals toward the southeast with lighter amounts northwest.
However, we are still awaiting observational data to confirm the
trajectory of the main storm core. This will play a part in just
how much snow we`ll see across the region. At this time, would
expecte to start seeing the effects to our SW around 03Z. To the
SE of a LBB/CDS line, the activity may be more convectively driven
--thus more intense. Most part 121 flights should do fine coming
 into KLBB this evening though how things look for the morning
 cargo and initial passenger departures is more nebulous as KLBB
 could see anything from a dusting to several inches. PIREPs
 through ATC or FSS are highly encouraged.



&&

.PREV DISCUSSION... /issued 252 PM CST Tue Feb 4 2020/

DISCUSSION...
Focus of this forecast remains around the winter storm impacting the
region through tomorrow.

Currently radar images show reflectivities across the southwestern
Texas Panhandle and the northern South Plains. We have reports of
accumulating snow nearing 3 inches in Friona and Dimmitt. Surface
temps from the West Texas Mesonet show the entire area is just at or
below the freezing mark so any liquid that does fall out of the sky
is expected to be snow from this point forward. CAMS indicate that
the southern Texas Panhandle and the northern South Plains will see
a few more hours of precipitation before a slow tapering off. Thus,
we have increased snow totals across the southern Texas Panhandle
and northern South Plains and have upgraded these areas to a winter
storm warning with accumulations of 3 to 6 inches expected.

Our stronger round of more widespread precipitaion will begin to
take hold this evening through tomorrow midday as embedded
disturbances round the base of the upper level trough and move
across the CWA. Overall we have made very little changes from the
previous forecast in terms of snowfall. Our heaviest snow bands are
still expected across the southern two-thirds of the CWA where
frontogensis is expected to be maximized in the 700 to 600mb layer.
This will result in storm totals of 7 inches southeast of a
Brownfield to Guthrie line. Outside of the heavy snow bands wide
spread accumulations of 2-5 inches is likely. We will continue both
the winter weather advisory and the winter storm warnings with no
changes. The other concerns for this winter event surrounds the very
cold low temperatures expected tonight and Thursday night along with
windchill values. Tonight, lows will drop into low teens and 20s
area wide with windchill in the single digits.

Our winter weather will begin to taper off midday tomorrow and by
tomorrow evening the trough axis will be right over the FA. In the
wake of the storm NW flow aloft will set up through Saturday with a
slow warm up in temperatures. Saturday a weak cold front will move
through the region dropping temperatures back into the upper 40s.
This will be followed by zonal flow to close out the weekend. Long
range models show another chance of precip mid week next week. /mlp

&&

.LUB WATCHES/WARNINGS/ADVISORIES...
Winter Storm Warning until 6 PM CST Wednesday for TXZ021>044.

&&

$$

99/99/26