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Forecast Discussion
NWS Omaha/Valley, NE


000
FXUS63 KOAX 241139
AFDOAX

Area Forecast Discussion
National Weather Service Omaha/Valley NE
539 AM CST Sun Jan 24 2021

...Updated Aviation Forecast Discussion...

.DISCUSSION...
Issued at 400 AM CST Sun Jan 24 2021

The main forecast concern is potential for heavy snow and areas of
blowing snow from late tonight into Monday night.

Some light precipitation is still possible over southeast NE early
this morning, otherwise we expect generally dry conditions the
rest of the day and into this evening. A weak ridge of high
pressure at the surface will build over the area today, turning
north or northeast winds to the east by evening.

Water vapor satellite imagery in combination with recent RUC/RAP
model initializations show a closed mid tropospheric low over
southern CA. That was not sampled well by any radiosondes in CA
last evening, or the past 24 hours. So feel that the models will
probably get a better handle on things with this mornings 12Z run.
Isentropic upglide/warm air advection will cause precipitation to
develop later tonight as the atmosphere saturates in the southern
parts of the forecast area. Forecast soundings suggest ice may
not dominate the column initially, so have mentioned some freezing
drizzle potential late tonight mainly for areas south of I-80.
Generally have used timing of a RAP13/ECMWF blend for development
of precipitation tonight/Monday morning.

Models are in good agreement that 500 mb 12 hour HFC (height fall
center) will track from northwest OK at 12Z Monday to eastern KS
at 18Z Monday and then to northern MO by 00Z Tuesday. This is a
favorable pattern for heavy snow across our area. Currently have
highest totals across southwest IA and southeast NE, with values
of up to 12 inches possible. Forcing for ascent will be strong
enough for potentially some periods of 1-2 inches of snow per
hour. 700 mb vertical velocities could reach 20 microbars/second,
especially for the southern parts of the forecast area. The GFS
indicated 850-700 mb mixing ratios/specific humidity values from
3 to 4 g/kg. With a 12 to 15 hour period of lift, that fits OK
with expected snow amounts. There is decent lift shown in the
dendritic growth zone, especially Monday afternoon. Headlines and
total snow amounts were well collaborated with bordering offices
and WPC. There is uncertainty on the northern edge of this system,
since there is likely to be a fairly sharp cutoff to the edge of
heavy snow. We errored on the side of caution, and added a Winter
Storm Watch to areas north of the Winter Storm Warning, where
there could still be moderate to heavy snow. Expect that amounts
will be revised several times as we get into the event. See maps
on our web page for areas where we are forecasting the highest
amounts, which are south of Omaha/Council Bluffs and Lincoln.
Deformation zone and lift decrease in the area by late Monday
evening, but we held onto headlines a little longer than that.

The system should be winding down late Monday night/Tuesday
morning, with the next system poised to the west. On Tuesday,
there could be some light snow around, but for now expect not much
accumulation with our area generally between systems. Lift starts
to increase west of our area by late afternoon, with the bulk of
vertical motion and decent moisture expected to move through our
area Tuesday night/Wednesday morning. That could produce another
1-3 inches of snow.

Expect highs today in the upper 20s/lower 30s north and in the 30s
south. Highs Monday will be in mid to upper 20s north and upper
20s or lower 30s south. Look for highs in the 20s Tuesday. Lows
will be mostly teens Monday night and Tuesday night, with mostly
single digits Wednesday night due to clear to partly cloudy skies
and light winds with fresh snowcover. We could see some spots drop
to below zero that night.

There is a weak system that could produce some light precipitation
Saturday, but we expect most of the Wednesday to Sunday timeframe
to be mainly dry. Temperatures should remain below normal
Wednesday and Thursday, then trend toward at or above normal for
Friday and Saturday.

&&

.AVIATION...(For the 12Z TAFS through 12Z Monday morning)
Issued at 531 AM CST Sun Jan 24 2021

MVFR and IFR criteria are expected over the TAF period. Ceilings
are expected to continue to lower and meet IFR criteria near 15Z.
The radar is also picking up on some light precipitation across
the area, but it seems it`s not reaching the surface.

Ceilings will briefly improve back to MVFR criteria after 20Z. As
the next system approaches, ceilings will again lower to IFR
criteria beginning near 03Z tonight. IFR criteria and inclement
weather are expect after that time. A Winter Storm Warning is
effect at KOMA and KLNK where significant snowfall is expected.

&&

.OAX WATCHES/WARNINGS/ADVISORIES...
NE...Winter Storm Watch from Monday morning through late Monday night
     for NEZ030>034-042>045.

     Winter Storm Warning from 3 AM Monday to 3 AM CST Tuesday for
     NEZ078-088>093.

     Winter Storm Warning from 6 AM Monday to 3 AM CST Tuesday for
     NEZ050>053-065>068.

IA...Winter Storm Watch from Monday morning through late Monday night
     for IAZ043-055-056.

     Winter Storm Warning from 6 AM Monday to 3 AM CST Tuesday for
     IAZ069-079-080-090-091.

&&

$$

DISCUSSION...Miller
AVIATION...Fajman

NWS OAX Office Area Forecast Discussion



Forecast Discussion
NWS Hastings, NE


000
FXUS63 KGID 241254
AFDGID

AREA FORECAST DISCUSSION
National Weather Service Hastings NE
654 AM CST Sun Jan 24 2021

...Aviation Update...

.DISCUSSION...(Today through Saturday)
Issued at 549 AM CST Sun Jan 24 2021

General overview of the entire 7-day forecast:
Another unusually-late discussion on an unusually-busy forecast
shift...our apologies. Fortunately, we have now whittled down the
sheer number of low pressure systems affecting our coverage area
with accumulating snow and/or ice from four (as it was in our
official forecast a few days ago) to now just two over the course
of the next 3 days, but needless to say the MOST IMPACTFUL one is
yet to come, as confidence continues to increase in a fairly heavy
snow event centered on the daytime hours Monday. In concert with
national QPF (liquid precipitation equivalent) guidance from WPC,
our forecast snow amounts for what is now System 1 late tonight
through late Monday night have spiked up to as much as 6-11" for
the majority of the CWA (but overall-highest southeast quadrant),
with only far northern counties currently slated to see lesser
amounts in the 3-5" range. Will trend the rest of the discussion
into more of a "bullet point" format to hit the main/key points,
and largely shy away from much in the way of "hardcore"
meteorological discussion/jargon:

- Quick look at recent/current weather as of 5 AM:
As it turned out, the somewhat-low confidence Winter Weather
Advisory in effect yesterday/last night ended up verifying quite
well, as indeed most of the CWA (all but mainly the far west) did
see enough freezing drizzle to slicken roads/cause issues. While
the freezing largely ended during the evening for most areas and
this Advisory was allowed to expire overnight, we were caught off
guard a little bit overnight by a pesky narrow snow band that
dropped a solid few tenths of an inch mainly near/north of the
Highway 6 corridor in Nebraska (models had earlier insisted that
this band should have largely fallen apart before it reached our
CWA. At any rate, it wasn`t a major overnight snow by any means,
and it has now finally dissipated, paving the way for what should
be a dry/uneventful daytime-early evening time frame for the vast
majority of the CWA. Under widespread low cloud cover, overnight
low temps are on track to bottom out mainly mid 20s. Behind a weak
cold front that helped scour out enough low level moisture/lift to
end the freezing drizzle threat, northerly breezes are currently
averaging around 10 MPH with some slightly higher gusts.

- Daytime hours today:
Truly the calm before the storm. Despite plentiful cloud cover,
do not anticipate any additional precip until at least a few hours
after sunset. Highs today aimed around 30 most areas, except for
mid 30s mainly in KS zones. Breezes generally no more than 10-15
MPH turning from northerly to easterly.

- System 1 (what is now expected to be a fairly major snow event
  peaking in intensity Monday daytime but starting overnight
  tonight and lasting into Monday night):
Only time will tell whether we are perhaps "overdoing it" for
especially parts of our northwest half, but at least for now our
forecast has largely bought into the most-aggressive snow amount
solutions that have been fairly consistently advertised by the
ECMWF for a few days now. For sure, confidence in amounts of
generally 6-11" is highest within mainly the southeastern quadrant
of our CWA as this is where various models includes NAM/GFS also
really pile on a fairly significant amount of snow in closer
proximity to what now appears to be a fairly potent closed mid
level circulation tracking across northeast KS. However,
confidence in 6+ inch amounts especially northwest of roughly an
Alma-Grand Island- Genoa line is admittedly a bit shakier, as the
cutoff between the heaviest/lightest amounts could end up being
tighter than what the current forecast reflects. But at any rate,
with the main snow event centered on Monday daytime only 24-36
hours away, it was decided to go ahead and upgrade/expand the
previously- issued Watch to a Winter Storm Warning for the vast
majority of the CWA. Only our far northern-most 3 counties
(Valley/Greeley/Nance) were placed in a Watch, as that area stands
the greatest uncertainty of seeing 6+" at this time. Of course,
these counties will almost surely be converted to either an
Advisory/Warning within the next 12-18 hours as confidence in
advertised snow totals hopefully further increases a bit. Once
again, the MAIN SHOW of this system will occur during the daytime
hours Monday, but have started the Warning at midnight mainly to
account for the leading edges of accumulating snow pushing in from
south-to-north, along with the possibility that a brief period of
freezing drizzle could precede any snow by a few hours as low-
level saturation/lift ramps back up slightly ahead of deeper mid
level saturation. In fact, technically, a bit of freezing drizzle
could even get back underway before midnight, particularly if the
NAM is onto anything. Following the main daytime snow event, the
compact/potent system will shift east across the IA area
overnight, allowing our snow coverage/intensity to really taper
off, but not necessarily completely end as residual lift between
systems could keep at least some areas of light snow/flurries
going. For sure though, the main event will be over well before
sunrise Tuesday, and the current Warning/Watch officially end at 3
AM Tuesday. Last but certainly not least, winds with this system
will not be "terribly" strong (not anticipating blizzard
conditions), but sustained northerly speeds of 10 to 20 MPH with
gusts to around 30 MPH during the peak of the storm will be
problematic enough to cause at least some modest blowing/drifting,
despite the snow tending to favor a bit denser/heavier nature
(not very fluffy). Winds will average lighter than this both at
the beginning and latter stages of the event.

- System 2 (a weaker, lighter snow event centered mainly during
  the Tuesday afternoon-overnight time frame):
Although arguments could be made that this is not the best
approach, at least for now we have decided to "ignore" this
secondary system in terms of the current Warning/Watch, as we
really want folks to focus on the first/Monday system above all
else. However, at some point, we may need to tack on a Winter
Weather Advisory to cover what currently appears to be generally
1-3" of additional snow with this last/final disturbance of the
work-week. Based on latest trends, this really appears to be more
of a Tuesday evening-night event than anything else, but certainly
the Tuesday daytime hours could at least feature areas of light
snow/flurries as well as the "break" in between systems is just so
brief. At least for now, we expect there "should" be enough mid
level saturation to keep freezing drizzle from being much of an
issue with this last system, but as always this bears watching.
Then again, with plenty of snow already on the ground in most
areas from System 1, the overall impacts of any sneaky freezing
drizzle will probably be minimized somewhat anyway.

- Wednesday-Friday (the QUIET/dry periods beyond the early-week
  chaos):
Unless some flurries are still hanging on in our far east right
away Wednesday morning, confidence remains high that these 3 days
will be dry as upper ridging takes hold.

- Saturday (dry for now but bears watching for at least a glancing
  blow):
Both the ECMWF/GFS suggest that our going dry forecast for
Saturday is not necessarily guaranteed as yet another fast-moving
disturbance crosses the Plains from west-southwest to east-
northeast. At least for now, these models keep precipitation just
BARELY south-through-east of our CWA, but obviously a LOT could
change between now and then.

- Temperatures Monday-Saturday:
High temps for especially Monday-Thursday just keep trending
colder (due in part to anticipated snow cover). It now appears
that Tues-Wed will be outright coldest with highs only around 20,
with a gradual rebound thereafter to mainly low 30s Thurs and
mainly low 40s Fri-Sat. However, as is always the case with fresh
snow cover, it could easily modulate temps by 5+ degrees depending
on how deep/persistent it is, so we`ll see just how aggressive the
late-week "warm up" might be. As for lows, Wed night is currently
the best candidate for coldest of the week as we currently have
all areas well down into single digits. However, if winds end up
being light and skies clear, could easily foresee some areas
realizing their first sub-zero readings of the winter given the
expected snow cover.

&&

.AVIATION...(For the 12Z KGRI/KEAR TAFS through 12Z Monday)
Issued at 653 AM CST Sun Jan 24 2021

Visibility/precipitation:
High confidence in dry conditions and VFR visibility through at
least the first 12-14 hours of the period. Beyond that, initially
light freezing drizzle and/or snow will gradually ramp up to a
steadier snow as the night wears on, and have visibility
tentatively falling to IFR levels for this.

Ceiling trends:
Sub-VFR expected/likely throughout the period. Much of the first
half will likely teeter very near the low-end MVFR/high-end IFR
breakpoint, but have leaned barely on the MVFR side for now.
During the latter half of the period tonight, high confidence in
IFR and eventually LIFR as precipitation moves in.

Winds:
No major issues to speak of as sustained speeds will average near-
to-below 10KT for most of the period, but start to creep up into
the sustained 10-15KT range very late (mainly after 08Z).
Direction gradually transitions from northerly this morning to
easterly to easterly this afternoon-evening and then starting to
trend back northeasterly late tonight.

&&

.GID WATCHES/WARNINGS/ADVISORIES...
NE...Winter Storm Warning from midnight tonight to 3 AM CST Tuesday
     for NEZ046>049-060>064-072>077-082>087.

     Winter Storm Watch from Monday morning through late Monday night
     for NEZ039>041.

KS...Winter Storm Warning from midnight tonight to 3 AM CST Tuesday
     for KSZ005>007-017>019.

&&

$$

DISCUSSION...Pfannkuch
AVIATION...Pfannkuch

NWS GID Office Area Forecast Discussion