Area Forecast Discussion
Issued by NWS Hastings, NE

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FXUS63 KGID 240238
AFDGID

Area Forecast Discussion
National Weather Service Hastings NE
938 PM CDT Sat Sep 23 2017

.UPDATE...
Issued at 938 PM CDT Sat Sep 23 2017

The difficult forecast continues, but I did make some wholesale
changes to part of the forecast, and this went into and through
portions of tomorrow since it all went together with the frontal
passage. The outflow/front has slowed tremendously, just reaching
Hastings, but still not through Aurora. With the latest high-res
and other 00z guidance, the 00z NAM seemed to have the best idea
with what may happen. It does, as I suspected, lift the boundary
back to the northwest slightly with the main front still to the
west of Lexington. Think this can be seen some in the wind field
and the dewpoint gradient (however slight it is). The NAM lifts
the outflow/front back to the northwest, warming and returning
southerly winds to portions of south central Nebraska before the
main front moves through during the day tomorrow. There are still
large areas where this could be wrong and some large errors in
Temps. As the remainder of the 00z guidance comes in, maybe they
might have a better handle on it and will allow the mid-shift to
make further adjustments.

Additionally low stratus has developed north and west of the
outflow/front impacting portions of south central Nebraska, this
may lift some if the boundary lifts back northwest.

Rain chances continue to be similar, did not make much change
there, mainly it was in the temperature/wind parts of the
forecast. Think rain chances will increase from west to east
Sunday into Sunday night.

UPDATE Issued at 714 PM CDT Sat Sep 23 2017

The forecast is tricky as the outflow from thunderstorms has
pushed the effective cold front eastward. As of 7pm it has just
went through Grand Island and will be approaching Hastings and
Franklin shortly. Have made some major changes to the forecast for
this evening/early overnight hours with temperatures in Lexington,
Kearney, and Holdrege in the 50s and 60s with northerly winds. Do
not have a good feeling for how deep this cold air is, and thus am
not sure how far east it will push. Scattered
showers/thunderstorms behind the outflow/front continue, but are
not as widespread as earlier...thus not reinforcing the cold air
as much. If the cold air is fairly shallow, the front may
retrograde back to the east, but if the cold air continues to be
reinforced, it could stay put...being quite a bit east of where
nearly all the models have it at this time. Adjusted the forecast
through midnight with these wholesale changes over the western
sections, trying to account for the eastward push of cold air/wind
direction change. Will continue to monitor trends and adjust
overnight with a later update.

&&

.SHORT TERM...(This evening through Sunday)
Issued at 141 PM CDT Sat Sep 23 2017

Extend and timing of rainfall across the CWA the
primary forecast concern along with temperatures.

Current water vapor imagery along with 12Z upper air analysis
showing high amplitude flow with riding in the eastern half of the
CONUS and troughing in the west. GOES-16 7.34 um water vapor channel
has a closed low spinning over SE Nevada. This system has been slow
in movement with little in the way of 500 mb height falls/rises in
the past 24 hrs. At the surface, a stationary front was bisecting
Neb from northeast to southwest, just clipping our NW CWA. This
front has lifted just a bit northward today. In addition to the
typical wind pattern associated with the front, dewpoints ranged
from the upper 60s south to near 50 just north of the front. These
were even lower in the Neb panhandle, upper 30s to lower 40s.
Similar differences noted in the surface temps. We did have a few
showers pop up in the eastern part of the CWA during the mid
morning, but coverage was isolated at best with no lightning noted.

Starting late this afternoon and into the evening, expect isolated
thunderstorms to pop up across the CWA. Would think the better
chances will still be across the western half of the CWA, closer to
the surface front. It does appear that there will be adequate CAPE
and deep layer shear to allow for a few strong/severe storms,
especially prior to sunset. However the best CAPE is in the warm
sector, aways from the better forcing to the west. Best effective
shear is west. Overnight all signs are pointing to the bulk of the
rain staying west of our CWA. the upper level system is progged to
move very little with several models taking the 500 mb low from SE
NV today to the UT/WY/CO border area by Sunday evening, then
gradually filling and lifting the trough northeast, with its
influence on us ending Tues evening. Under this scenario much of the
precip should be along and just behind the surface front. Starting
times for the persistent rain should be around sunrise Sunday in the
far western CWA and not until Sunday evening in the east. Of course
there will be pop up warm advection type showers/tstms ahead of the
main system but this should not be as widespread. Severe storms are
not anticipated with the main precip shield Sunday into
Tuesday.

.LONG TERM...(Sunday night through Saturday)
Issued at 141 PM CDT Sat Sep 23 2017

Continuing on with the short term discussion, expect
rain to continue into Tuesday, with the best chance across the CWA
Sunday into Monday afternoon. Again this is all tied to the upper
level system which has continued to slow a bit from what was
expected 5 days ago. Given the anticipated movement of the upper
level system, the surface front should begin moving east during the
day on Sunday and exit the CWA before sunrise Monday. However, the
post frontal rain should continue Monday, slowly ending from west to
east Tuesday.

After that, surface high pressure will move into the Plains drying
things out. Cooler air is expected to remain over the region and
highs for the rest of the work week with highs some 5 to 10 degrees
below normal for this time of year.

&&

.AVIATION...(For the 00Z KGRI/KEAR TAFS through 00Z Sunday)
Issued at 714 PM CDT Sat Sep 23 2017

This is a tricky aviation forecast. Outflow from the thunderstorms
as pushed the effective cold front eastward. Winds have switched
at both terminals as of this discussion to west/northwesterly and
expect them to continue to be north northwesterly through the next
few hours. All of guidance keeps the frontal position further
west, and confidence is low on how this will evolve. This is
important for the aviation forecast for wind direction. If the
cold air is more shallow, the front could push back to the west
and southerly winds could return. For now, did not make wholesale
changes to the forecast beyond 6 hours, so allowed the return of
southerly winds to the TAF for the overnight hours. This may not
happen, and will amend with later trends. In addition to the
surface wind direction concerns, low level wind shear will be a
concern at both terminals overnight.

Some scattered low ceilings may impact the terminals tonight, but
overall, expect VFR conditions to prevail as upstream sites have
VFR conditions. Ceilings are expected to decrease over the next 24
hours as the frontal position continues to move eastward. Some
MVFR/IFR ceilings are possible overnight, but are more likely
during the afternoon hours Sunday.


&&

.GID WATCHES/WARNINGS/ADVISORIES...
NE...None.
KS...None.
&&

$$

UPDATE...Billings Wright
SHORT TERM...Ewald
LONG TERM...Ewald
AVIATION...Billings Wright


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