Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

FXUS63 KGID 121731

Area Forecast Discussion
National Weather Service Hastings NE
1131 AM CST Tue Dec 12 2017

.SHORT TERM...(Today and tonight)
Issued at 418 AM CST Tue Dec 12 2017

Upper air and satellite data continue to show a well amplified 
pattern across the CONUS early this morning. Troughing from a low 
pressure system over Ontario extends south through the Midwest and
toward the Gulf Coast region, while ridging remains over the West
Coast. As a result, northwesterly flow continues across the 
Plains, and with a lack of notable disturbances moving through, 
it's been a dry night. At the surface, winds have continued to 
diminish overnight with a weak pattern in place, and are currently
west- northwesterly around 5-10 MPH. Much of the CWA has clear 
skies, but a stubborn narrow swath of lower level N-S oriented 
stratus continues to hang around central areas. 

The forecast through the short term period remains dry, and 
compared to yesterday, Wed and Thurs, today is going to seem nice.
Aloft, not looking at much change in the overall northwesterly 
flow, though later tonight, another quick moving shortwave 
disturbance will be making its way into the region. The main 
impact on this will be another reinforcing shot of northwesterly 
winds as a front gets pushed south into the region, after midnight
tonight. Winds will remain westerly today, and the main reason 
why it'll be different than Mon/Wed/Thur is the speeds, which are 
forecast to top out in the 10-15 MPH range. This narrow band of 
stratus looks to stick around a while longer, but will eventually 
diminish, and have mostly clear skies once it does. No big change 
to forecast highs for today, which range from the upper 40s in the
far east to mid 50s in the west.

.LONG TERM...(Wednesday daytime through Monday)
Issued at 418 AM CST Tue Dec 12 2017

General overview of this 6-day period: 
Overall, no "big" changes in this forecast since 24 hours ago. In
short, this means that the vast majority of this time frame looks
to continue our incredibly dry stretch, but perhaps with a few 
opportunities for light precipitation (maybe even a touch of 
accumulating snow?). Temperature-wise, the entire period will also
continue our recent above-normal regime, with 4 of these 6 days 
currently progged to feature high temps at least 10-15 degrees 
above mid-December normals. 

Likely/potential weather hazards:
As for issues that are/could eventually be worthy of inclusion in
our Hazardous Weather Outlook, we cannot rule out light wintry 
precip eventually becoming an issue here or there, but confidence 
in occurrence/timing remains far too low to highlight at this 
point. As a result, the only higher-confidence issues look to 
occur right away Wednesday with another round of strong northwest 
winds and (mainly in our south) near-critical fire weather. Please
refer to the separate FIRE WEATHER section section below for more
details. Thursday looks to be fairly windy as well, but probably 
not quite as strong as Wednesday, and thus have kept this out of 
the HWO for now until/unless speeds trend upward. 

Basic upper air/surface weather pattern:
The majority of this time frame will feature a continued
northwest-flow regime in the mid-upper levels. However, at least 
a brief transition to more southwesterly flow looks to occur 
Saturday-Sunday in association with a passing shortwave trough. 
This particular weekend system will be the 3rd of three distinct 
disturbances expected to cross the Central Plains over the course 
of these 6 days, the latter two of which could result in at least 
light/pesky precip. At the surface, each of these 3 waves will 
drag through relatively weak cold fronts (temperature-wise) but 
with breezy-to-windy north-northwest winds (especially Wed- 
Thurs), while days without these frontal passages will feature 
comparably lighter westerly breezes (such as Friday). 

Basic Precipitation overview:
The official forecast now contains two windows of opportunity for
what is currently expected to be light amounts of rain and/or 
snow: 1) Thursday mainly within the eastern half of our coverage 
area (CWA)...2) Saturday night mainly in our far northeast 
counties (but if the latest ECMWF is onto anything this could 
perhaps be more widespread). 

Basic temperature overview: 
At least a modestly above-normal regime remains firmly in place 
for these 6-days as a whole. Four of the 6 days look to feature 
rather uniform highs mainly ranging between the upper 40s-upper 
50s. Confidence remains fairly high that the 2 coldest days will 
be Thursday and Sunday. Right now Thursday features highs into the
low-mid 40s most areas, but do not be surprised if especially 
some north-northeast areas don't escape the 30s. Of all six days, 
the "biggest" change was to Sunday with highs lowered generally 
2-3 degrees mainly into the mid-upper 40s, and model guidance 
suggesting that further downward trending could occur. Overnight 
lows looks to mainly range from mid-20s to near 30, with the 
exception of Thursday night with colder lower 20s. 

With all the big-picture points covered, will finish with some 
daily-specific details... 

Wednesday/Wednesday night:
It's actually eerily similar how Wednesday resembles yesterday 
(Monday)! Like Monday, the day breaks with a compact-but-potent 
trough diving quickly southeast across the NE/IA border area. 
Following true to form, the big story at the surface will be 
another round of strong northwest winds behind the attendant cold 
front, and have again bumped up speeds, now calling for several 
hours of sustained near 30 MPH/gusts near 45 MPH (and likely 
locally higher). For now this looks to fairly confidently be a dry
frontal passage. High temps are also aimed extremely similar to 
those of Monday, ranging from upper 40s north, low 50s central, 
mid-upper 50s south. 

Thursday/Thursday night:
Right on the heels of the Wednesday wave, another somewhat 
larger- scale shortwave dives southward through the Central 
Plains, bringing a reinforcing shot of cooler air and likely 
making this the overall-coldest day of these long term periods. It
will again be quite breezy-to-windy, but at least for now speeds 
look to average roughly 5 MPH "lighter" than Wednesday. The 
biggest uncertainty/question marks surround precip potential. The 
official forecast now mentions potential for rain and/or snow 
showers especially within our eastern half, and although not 
reflected in the official forecast, a few tenths of an inch of 
snow accumulation probably cannot be ruled out, especially if 
temps trend down a bit more. At this particular moment, the 
ECWMF/GFS are more aggressive than the NAM with precip potential. 
For now the Thursday night period remains dry, but could see a bit
of activity lingering into the evening. 

Friday looks to be the overall "nicest" of these 6 days with
lighter westerly breezes and temps rebounding at least 5-10
degrees from Thursday. Saturday appears fairly mild as well, but
changes will again be underway as the next cold front moves
through. For now the daytime period remains dry. 

Saturday night-Sunday:
The strongest, most organized of the 3 disturbances passes 
through the Central Plains through this time frame, bringing a 
cool down behind Saturday's initial frontal passage. The million 
dollar question (and a rather murky one at that still being 5 days
out), is whether especially Saturday night could feature some 
light snow accumulation? Right now the ECMWF favors this more than
the GFS, but this time frame does bear watching. 

Sunday night-Monday:
For being 6-7 days out, confidence is currently high in a dry
period here as northwest flow aloft returns in the wake of the
weekend disturbance. Temps expected to rebound 5-ish degrees 
versus Sunday. 


.AVIATION...(For the 18Z KGRI/KEAR TAFS through 18Z Wednesday)
Issued at 1131 AM CST Tue Dec 12 2017

Significant Wx: LLWS 07Z-15Z then NW winds gust 35-40 kts.

This Afternoon: VFR. No clds at or below (AOB) 10K ft. W winds
6-12 kts. Confidence: High

Tonight: VFR. No clds AOB 10K ft. LLWS develops 06Z-08Z. W winds
8-15 kts increasing and becoming gusty toward dawn. Confidence:

Wed thru 18Z: VFR. No clds AOB 10K ft. NW winds increase 22-30 kts
with gusts 35-40 by 15Z. Confidence: High


Issued at 418 AM CST Tue Dec 12 2017

Regarding fire weather concerns for Wednesday:
In many ways, Wednesday's weather situation strongly resembles 
that of yesterday (Monday), with strong northwest winds sustained 
generally 25-35 MPH/gusting to 45+ MPH at times, and with rather 
low relative humidity (RH) in our southern zones. However, at 
least for now, the main difference for Wednesday is that 
dewpoints/resultant RH values look to hold a little higher. As a 
result, the latest official forecast features no outright- 
critical RH values of 20-percent-or-lower, and only a fairly 
limited area of near-critical RH of 22-25% mainly southwest of a 
Cambridge-Beloit line (anywhere northeast of this line is expected
to see progressively higher afternoon RH with northward extent). 
So, all this being said, unless RH trends lower (worse) in later 
forecast packages (which IS always a possibility), we currently 
cannot justify the issuance of any fire weather Watches/Warnings 
for Wednesday afternoon, but will certainly continue to highlight 
near-critical wording in the Hazardous Weather Outlook, especially
given the strong winds and dry vegetation. 




National Weather Service Glossary of Abbreviations