Area Forecast Discussion
Issued by NWS Hastings, NE

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533
FXUS63 KGID 230000
AFDGID

Area Forecast Discussion
National Weather Service Hastings NE
700 PM CDT THU SEP 22 2016

.UPDATE...
Issued at 649 PM CDT Thu Sep 22 2016

Updated the forecast tonight to expand the area of fog mentioned
tonight to include some areas along and just south of I-80 from
near Kearney and farther west where stratus stuck around longer
today. Am contemplating hitting the fog a bit harder than we have
in the forecast, judging by later HRRR runs.

&&

.SHORT TERM...(This evening through Sunday daytime)
Issued at 417 PM CDT Thu Sep 22 2016

Forecast highlights/overview of these next 72 hours:
The primary concern through these 72 hours centers around the
still-somewhat- unclear rain/thunderstorm chances centered on the
Friday night- Saturday night time frame (rain chances/PoPs have
been held below "likely" 60+ percentages at this time due to
inherent uncertainty in coverage). Assuming that at least
isolated/scattered activity is able to develop during this time
frame, a few strong to severe storms are not out of the question,
especially Saturday afternoon within eastern parts of the CWA.
Turning back to the nearer-term, have left the forecast void of
any mentionable (meaning 15+ percent) shower/storm chances through
tonight, but these chances are not necessarily "zero" either.
There is some concern for at least patchy fog development mainly
within counties along/especially north of I-80 tonight, with a few
higher-res models such as the HRRR suggesting that this fog could
be a bit more widespread/impactful. For now, have not gotten very
aggressive but did at least introduce a mention of "patchy fog
with visibility less than 1 mile" to tonight`s forecast.
Temperature-wise, unlike today, Friday is expected to have a more
uniform temperature regime with well-above normal heat in all
areas. Then, a gradual cool-down should occur over the weekend,
with highs by Sunday actually slightly below normal (and roughly
20 degrees cooler than Friday).

Taking a look at the current scene as of 330 PM (and as outlined
in the "update" discussion earlier this morning)...sky
cover/temperature forecasting has been a bit of an "adventure"
today thanks to a stark division between widespread sun/heat over
southern/eastern counties and a nearly solid deck of low stratus
clouds over many northern/western counties. As expected, the
southern/eastern edges of this low stratus has in fact slowly
retreated/eroded off to the north and west in response to daytime
heating and mixing. Gave a "best shot" at where actual high temps
will end up, but currently it appears a rather big gradient from
low-mid 70s far north/west-central...mid 70s to low 80s in the
central Tri-Cities area...and noticeably hotter upper 80s to low
and even mid-90s in far eastern and southern counties (hottest of
all in our southern-most KS zones). At the surface, the edge of
this stratus is fairly closely tied to the position of a nearly-
stationary front that is draped across the CWA from west-southeast
to east-northeast. Fairly light north/northeast breezes prevail
north of this boundary while southerly breezes prevail to its
south...and a hodge-podge of light/variable in between. In the
mid-upper levels, a period of weak/subtle ridging appears to be in
control of the Central Plains between a departing shortwave over
IA, and a powerful/highly-amplified trough over the western CONUS
featuring a deep closed low centered over NV.

Now looking ahead forecast-wise through these next 6 day/night
forecast periods...

This evening/tonight:
Despite various hints in various models that at least spotty
shower/thunderstorm activity could at least flirt with the fringes
of our CWA (especially northern areas after dark), decided that
the overall coverage of any of this possible activity within our
borders would remain closer to 10 percent than 20 percent, and
thus these chances are below mentionable thresholds (in other
words, the forecast reads as "dry" but please note that chances
are not necessarily zero). Certaintly, the most legitimate
rain/storm chances tonight should focus north of our CWA from
northern NE eastward into IA, where mid level temps are a touch
cooler and where there is better convergence near the nose of a
modest low level jet. At the surface, the aforementioned front
draped across the heart of the CWA will slowly lift northward as
a warm front overnight in response to low pressure deepening to
the west, turning breezes more southerly as it does so. It is
still unclear exactly what the low stratus deck does, as its
northward-retreat is likely to stall out for a time around
nightfall, keeping it stubbornly over our northern/west-central
zones well into the night before eventually vacating our CWA for
good. As mentioned in the opening paragraph, there are hints of
fog development overnight mainly in counties north of the I-80
corridor, and have introduced a generic "patchy with visibility
less than one mile" to get the ball rolling this direction. Temp-
wise, if anything nudged down lows very slightly, aiming for mid-
upper 60s most areas but low 60s far north.

Friday daytime/night:
This is "part 1" of our upcoming rain/storm chances, and
confidence is pretty high that they will not even arrive in
western zones until after dark. In the mid-upper levels, the
aforementioned deep western trough will gradually approach/invade
the Central Plains, with its primary mid level vort max reaching
western WY by late afternoon, and into southeast MT by daybreak
Saturday. Ahead of this system, confidence remains rather high in
a dry, mostly sunny and very warm (some would say hot) day with
noticeably breezy southerly winds overtaking the entire CWA as the
warm front blasts well to our north. Made little change to high
temps/winds, as the entire CWA is expected to top out 88-91 in the
presence of sustained south winds 15-25 MPH and gusts 25-25 MPH,
highest in the afternoon. Then, as the night wears on, forcing
gradually increases from the west, and fairly low chances for
isolated to perhaps scattered of showers/thunderstorms arrive to
the western CWA before midnight and then expand CWA-wide late in
the night. Can`t completely rule out a rogue strong to marginally
severe storm mainly with hail, but this should be the exception.
The invading north-south oriented synoptic cold front may be right
on our western doorstep by night`s end, but ahead of this boundary
low temps should hold up mid-upper 60s most areas.

Saturday daytime/night:
This is "part 2" of the storm chances, but confidence in areal
coverage of this rainfall is still not high enough to justify
"likely" PoPs within the CWA. Certainly though, chances are higher
in southern/eastern counties than they are in northern/western
counties (which could really end up missing out on things). In the
mid-upper levels, the main vort max swings north toward ND during
this time, while the large-scale trough trailing to its south
slowly edges east through the Plains. Meanwhile at the surface,
barring some fairly big changes, models have seemingly settled on
a steady west-to-east progression of the synoptic cold front
through the CWA during this time. Based on instability/shear progs
and the timing of the boundary, especially our eastern-most
counties may see at least a brief severe storm threat Saturday
afternoon along this front, and agree with the SPC putting these
areas in a Marginal Risk. This will bear watching, because if the
front slows at all, then more of our CWA may be in a severe risk
mainly for large hail/wind. Taking the latest models literally
(especially the NAM) suggest that the majority of shower/storm
chances may be over with by sunset. However, models such as the
ECMWF/GFS are slower with precip departure, and thus will linger
modest PoPS through the night especially in eastern/southern
zones. High temps are aimed low 80s most areas and with With
cooler air invading, lows expected to drop into the 50s nearly all
areas.

Sunday daytime:
Although it could be a close call in extreme southeast zones,
confidence was high enough to leave the entire CWA void of
shower/storm chances behind the departing cold front. As a result,
we are looking at a "fallish" day featuring plenty of sunshine,
cooler temps and breezy northwest winds averaging 15-20 MPH with
higher gusts. If anything, nudged up highs 1-2 degrees given
expectation of sun/decent mixing, aiming for 69-72 most areas.

.LONG TERM...(Sunday night through Thursday)
Issued at 417 PM CDT Thu Sep 22 2016

Fall-like conditions are forecast to continue on into the beginning
of the next work week, with temps climbing back to normal by mid-
week.

The main change made to the forecast for the long term was to remove
precipitation chances. At the start of the period, models are in
pretty good agreement showing the main upper level trough axis
continuing to push through/east of the CWA. By 12z Monday, the axis
looks to extend from central MN south through central KS, with
northwesterly winds in place across the CWA. That northwesterly flow
aloft remains in place through at least Mon/Tues, as ridging over
off the west coast starts pushing east through the Rockies. As we
get into Wed/Thurs, that ridging moves onto the Plains/right over
the CWA. The 12Z run of the ECMWF/GFS are in good agreement showing
little in the way of disturbances moving through, keeping this dry.
The 12Z run of the Canadian is an outlier, keeping a massive cut off
lower pressure system over the central CONUS through the period.

At the surface, high pressure builds into the region behind the
weekend frontal passage, with light, northerly winds on Monday
turning more variable for Tuesday. Return southerly flow looks to
return for mid-week. As far as temperatures go, not expecting a lot
of change for Mon/Tues compared to Sun, with highs generally in the
mid-upper 60s and dewpoints in the 30s and 40s. Warmer temps
forecast for mid-week with the upper ridge building in, reaching the
mid 70s by Thursday.

&&

.AVIATION...(For the 00Z KGRI/KEAR TAFS through 00Z Friday evening)
Issued at 649 PM CDT Thu Sep 22 2016

Biggest issue will be low-level wind shear overnight as a low-
level jet develops. We will be on the northern edge of concern.
Also, visibility may lower as we were on the southern edge of
where stratus stuck around for much of the day, so confidence is
really not high here.

&&

.GID Watches/Warnings/Advisories...
NE...None.
KS...None.
&&

$$

UPDATE...Heinlein
SHORT TERM...Pfannkuch
LONG TERM...ADP
AVIATION...Heinlein



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