Area Forecast Discussion
Issued by NWS Hastings, NE

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FXUS63 KGID 140951
AFDGID

Area Forecast Discussion
National Weather Service Hastings NE
451 AM CDT Thu Sep 14 2017

.SHORT TERM...(Today and tonight)
Issued at 451 AM CDT Thu Sep 14 2017

West-northwest flow remains in place early this morning across the
region, driven by a weak ridge of high pressure extending
northeastward from NW Mexico, which is set up between low pressure
over southern CA and the remnants of Irma sliding into the Midwest.
Between a subtle wave moving through the region and lower/mid level
warm air advection, have been seeing scattered showers and
thunderstorms around through the overnight hours, most of which are
currently affecting portions of eastern NE/KS. At the surface,
generally southerly winds remain in place across the CWA, with low
pressure sitting over western SD and a trough axis extending south
into NE and KS. Temps at 3 am are warmer than 24 hours ago, ranging
from the lower 60s to near 70.

Confidence in precipitation chances through the short term, esp
during the daytime hours, isn`t high. Am keeping some slight chance
PoPs in the forecast through the mid morning hours, with many models
showing that potential. Decided to keep the remainder of the daytime
hours dry, but some models try lingering some activity through
midday. Will let day shift monitor how things evolve and adjust as
necessary, not convinced things would last that long. Through the
daytime hours, the area of low pressure currently over southern CA
pushes east, weakening some as it moves into the Rockies this evening
and another system digs south through the Pac NW. A shortwave
disturbance out ahead of this system in the Rockies looks to bring
additional chances for thunderstorms to the CWA, mainly this
evening/overnight. Instability values area shown by some models to
be a bit better than the past few days, but deeper layer shear
remains weak, so not expecting severe weather.

The gradual warming trend in temps continues today. Highs reached
the upper 80s to lower 90s yesterday, and today`s highs are forecast
to reach the 90s CWA-wide, with upper 90s in the far southwest. That
surface trough axis will remain a player through the day, keeping
winds south-southwesterly. Speeds across roughly the northwestern
half of the CWA (closer to the axis) look to top out between 10-15
MPH, across the southeastern half, sustained speeds of 15-20 gust 25
MPH will be possible. With the gusty winds, hot temps and lower RH
values, fire weather remains a concern. See the fire weather section
below for more information.

.LONG TERM...(Friday daytime through Wednesday)
Issued at 451 AM CDT Thu Sep 14 2017

General overview of this 6-day period (next few paragraphs):
Taking a big-picture view, overall little change versus the
previous few forecast packages. This means that there are still a
multitude of relatively low-confidence rain/thunderstorm chances
(PoPs) on nearly every day/night (at least through Tuesday).
However, due in part to this low-confidence nature, those watching
recent forecasts closely will note that some days/nights have been
(and unfortunately will likely continue to) "flip-flop" whether or
not they feature mentionable PoPs, as models continue to struggle
with exact timing/placement of the best forcing/instability. There
are probably at least a few opportunities for severe storms
lurking within this time frame as well. At least for now, the only
"official" Marginal Risk from SPC is still focused on Friday
evening mainly within the northern half of our CWA, but depending
on cold frontal timing would not be surprised to eventually see
additional Marginal Risk-type setups materialize for Saturday
afternoon-evening (at least near the eastern edges of our CWA) and
then perhaps again early next week as the front lifts back north
as a warm front.

Temperature-wise, very little change from the previous forecast
here, and as usual overall-higher confidence than rain chances.
Friday still looks to be the outright-hottest day of these 6 with
highs aimed into the low-mid 90s nearly all areas, although there
is an outside chance that any stubborn/lingering daytime
convection could temper the heat a bit in a few spots (too soon to
say whether this might happen). Saturday then remains by far the
biggest question mark temp-wise, as models continue to gradually
converge on the timing of a fairly pronounced cold front sweeping
southeastward through the CWA over the course of the day. It`s
very possible that our official forecast highs may not be cool
enough in the northwest/warm enough in the southeast, but even as
it currently stands, we are calling for a healthy 20-degree
gradient ranging from 70-ish far north/west-central to 90-ish far
southeast, with the Tri-Cities area roughly 75-80. In the wake of
the front, Sunday is then the overall-coolest day with highs mid-
upper 70s Nebraska and low-80s KS. Thereafter, a return to
slightly above-normal heat commences for early-mid next week, with
highs rebounding back into the 80s all areas (possibly pushing 90
far south).

In other departments, although the fire weather concerns for
Friday do not appear as high/widespread in our southern zones as
they do for this afternoon, a few of our extreme southwest
counties could still meet "near-critical" thresholds. For further
discussion of this please refer to the separate FIRE WEATHER
section below.

General overview of the large-scale weather pattern:
The majority of this 6-day period looks to feature predominantly
southwest flow aloft, with a number of shortwave
troughs/disturbances passing through the Central Plains, albeit
most of them of the subtle/weaker variety. Very basically, the
Friday-Saturday night time frame will be prone to the strongest
large-scale forcing as a large scale trough gradually lifts out of
the northwest CONUS into southern Canada, giving a glancing blow
to our area as it does so. Thereafter, a period of relatively
"flat", more zonal flow develops for a few days, before more
organized disturbances potentially arrive by the Tuesday and/or
Wednesday ahead of the next larger-scale trough taking shape over
the western CONUS.

Will finish with some day-to-day comments, again emphasizing the
below-average confidence in the timing/placement of rain chances,
especially beyond Saturday.

Friday daytime-night:
Small shower/thunderstorm chances have been re-introduced/expanded
to much of the CWA for the morning hours, as models such as the
NAM in particular are showing lingering, likely non-severe
elevated convection working across the area from west-to-east.
Overall though, this should be breezy and seasonably hot day with
south winds sustained around 20 MPH/gusting up to around 30 MPH in
some places. While severe, near-surface based convection appears
fairly unlikely before 00Z/7PM, the evening/overnight could
feature an uptick in storm potential as the cold front edges in
from the north, with a few severe storms certainly possible mainly
north of Highway 6.

Saturday daytime-night:
As covered in the temperature section, look for a pronounced
north-south gradient as breezy north-northwest winds invade behind
the cold front. While most of the CWA is probably dry most of the
day, it seems that some afternoon storm potential (maybe even
severe?) could exist during the late afternoon/evening especially
in/near our far eastern zones along the passing front. Would not
be surprised to see SPC shift the Day 2 Marginal Risk back
westward a bit closer to our CWA on later updates, especially if
frontal timing slows any.

Sunday-Sunday night:
The day should be fairly quiet/pleasant/cooler in the wake of the
front, but already Sunday night shower/thunderstorm chances return
as the front starts returning northward as a warm front. Mainly
per the GFS, we have PoPs as high as 50 percent going here, but
this may prove overdone as the ECMWF keeps most of the convection
east of us.

Monday-Wednesday:
In a nutshell, warming back up with periodic rain/thunderstorm
chances, but low-confidence in the details for sure. While most of
this time frame should largely be dry, the GFS seems most
aggressive at pushing a disturbance across our general area from
the southwest Monday night-Tuesday time frame. Plenty of details
to work out here for sure.

&&

.AVIATION...(For the 06Z KGRI/KEAR TAFS through 06Z Friday)
Issued at 1227 AM CDT Thu Sep 14 2017

VFR conditions remain in the forecast for this TAF period.
Currently don`t have any mention of precip in, though models show
the potential for isolated development at times. No confidence in
location/timing of any activity. Kept the mention of LLWS in for
at least a few more hours early this morning, though it looks to
be marginal. Winds will again be southerly through the period,
increasing to around 15 MPH through the afternoon.

&&

.FIRE WEATHER...
Issued at 451 AM CDT Thu Sep 14 2017

Regarding "near-critical" fire weather possible mainly in our KS
zones mainly this afternoon and to a lesser-extent Friday
afternoon:

This afternoon:
Near critical fire weather conditions are forecast this afternoon.
As mentioned above, temperatures climbing into the 90s is expected
across the entire area, with some locations in the southwest
getting into the upper 90s. The main area of concern lies roughly
along/south of a line from Hebron NE to Phillipsburg KS. While
gusty winds will be possible across much of the CWA today, this
southern area looks to have sustained speeds near 20 MPH and gusts
near 25 MPH. Dewpoints CWA- wide are expected to remain mainly in
the 50s, with the lowest values in that southern area in the
lower 50s, but wouldn`t be too surprised if a few upper 40s work
in mid afternoon. Relative humidity values are currently forecast
to bottom out in the lower 20 to lower 30 percent range, again
with those lower values in the south. It`s possible that a few
locations could hit the 20%/20 MPH sustained/25 MPH gust criteria
for an hour, but at this point am not forecasting it to reach the
3 hr criteria needed for a headline, but will keep mention going
in the HWO. Day shift will be monitoring temp/dpt trends closely.

Friday afternoon:
Breezy south winds should be more widespread within the CWA than
today, with sustained speeds in most areas around 20 MPH and gusts
25-30 MPH. However, thanks largely to a modest increase in low-
level moisture/dewpoints, RH values are expected to hold up
generally 5-10 percent higher than Thursday. Because of this, only
portions of our extreme southwestern counties
(Furnas/Phillips/Rooks) appear at risk to meet near-critical
thresholds. So overall, not a "zero-risk" fire weather day by any
means, but at least a bit higher RH than today.

Closing with a quick review of our local fire weather criteria:
"Critical" means the 3+ hour overlap of relative humidity (RH) of
20-percent-or-lower and sustained winds/gusts of 20+MPH/25+MPH
(in the presence of sufficiently-dry vegetation/fuels). "Near-
critical" means the overlap of 25-percent-or-lower RH and
sustained winds/gusts of 15+MPH/20+MPH.

&&

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

$$

SHORT TERM...ADP
LONG TERM...Pfannkuch
AVIATION...ADP
FIRE WEATHER...ADP/Pfannkuch



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