Area Forecast Discussion
Issued by NWS Hastings, NE

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713
FXUS63 KGID 260958
AFDGID

Area Forecast Discussion
National Weather Service Hastings NE
458 AM CDT Wed Apr 26 2017

.SHORT TERM...(Today and tonight)
Issued at 457 AM CDT Wed Apr 26 2017

With little if any potential for precipitation through the
period (save for some pesky light drizzle this morning), the
number one story involves a respectable chance for at least patchy
frost formation and/or slightly sub-freezing temperatures across
various parts of our county warning/coverage area (CWA) late
tonight into Thursday morning. While not a "slam dunk" for frost
formation everywhere due mainly to some uncertainties regarding
cloud cover, felt it was prudent to play it safe and issue a CWA-
wide Frost Advisory on this shift to raise awareness another
level. Otherwise, the other main challenge involves a tricky cloud
cover forecast, with some models noticeably quicker than others in
eroding the low stratus deck today from west to east, which will
obviously have an implication on temperatures as well. Overall
though, today offers the best hope of the next several days for at
least parts of the CWA to see appreciable sunshine.

Taking a look at the current/recent weather scene as of 09Z/4AM:
Overall the late night largely playing out as anticipated here 24
hours ago, as the entire CWA remains socked in under a widespread
low stratus deck, and seeing chilly northwest winds sustained
generally 15-20 MPH/gusting up to around 30 MPH. With lows
ultimately expected to bottom out in the 33-37 range most areas,
we can legitimately say there is a wind chill factor this morning
with most folks heading out the door greeted by chills in the low-
mid 20s. Some models ending up being grossly overdone with rain
development Tuesday evening as there was hardly any at all, but
then overnight there has been at least one small surprise, as
both radar and occasional ground-truth observations (including
right here at the WFO) has indicated the presence of at least
patchy drizzle across various parts of the area (not really sure
why this drizzle waited until late in the night to kick in, as the
low cloud deck has been in place since yesterday?). Turning to
the big picture of the mid- upper levels, water vapor satellite
and short term analysis depicts a large- scale, slow- moving
trough axis crossing the central CONUS. For the most part though,
our local area is split in the middle between the primary mid
level waves/vort maxes, with one currently centered over the
Dakotas and helping force widespread precip especially over MN,
and another wave farther south currently swinging out of NM into
the TX Panhandle, and helping drive widespread convection across
parts of OK/MO/AR. Here in the "middle" between these main waves,
we are left void of mid level saturation but stuck with the low
clouds and (for some) light drizzle. On the surface weather map,
our gusty northwest winds are being driven by a decent pressure
gradient between a 996 millibar low near the IA/WI border area,
and high pressure building in from the northwest.

Now looking ahead forecast-wise through these next 24 hours or
so...

Today (including early this morning):
First off, don`t have a great feel for how much longer any light
drizzle might last or even if it`s very widespread at all (local
airport obs are showing little if any visibility reduction and
radar only sees drizzle within a short range due to being such as
low cloud phenomenon), but will continue a generic "patchy
drizzle" wording all areas through 7AM for now and re-evaluate
later whether it needs to go longer. Model low-level relative
humidity progs do support a gradual drying-trend within the
stratus layer over the next few hours so this may be enough to
shut any drizzle off. Getting into the daytime hours, again the
biggest challenge is determining how aggressively low clouds clear
(or how stubbornly they linger), and the resulting impact on high
temps. On the most optimistic side of things one have models such
as the NAM, which steadily vacates low clouds from west-to-east
and actually has all but the far northeast CWA clear by early
afternoon. On the flip-side, often more-reliable very short term
models such as the RAP/HRRR are at least a few hours slower with
west-to-east clearing potential, and suggest that as much as the
east-northeast half or so of the CWA could remain relatively
"socked in" even still by late afternoon. Admittedly just do not
have a great feel for this, so the official forecast plays
somewhat of a middle ground, but if anything leans more optimistic
for all areas seeing at least a few hours of sun (especially
west), with eastern areas perhaps not seeing much until pretty
late in the day. It makes sense that a good chunk of the area
should see at least "some" sun with the large-scale trough axis
passing east of our CWA with time. Temp-wise, obviously cloud
trends could easily result in 5+ degree errors, but will aim for a
modest little gradient from near-50 far north/northeast to upper
50s far southeast. However, if the cloudier solutions end up
verifying then more of our north/east would see upper 40s at best.
Even for those areas that see sun, northwest breezes will
continue to put a chill in the air, although the strongest speeds
with gusts to around 30 MPH will occur this morning, before a
slow/gradual decrease in speeds takes place this afternoon as the
pressure gradient eases up, with late afternoon gusts easing down
closer to 20 MPH.

This evening/tonight:
The main focus remains on frost/freeze potential, but the CWA-wide
Frost Advisory valid late tonight into early Thursday morning is
not a "slam dunk" to verify all areas mainly given some cloud
cover uncertainties. In the mid-upper levels, forcing remains
fairly minimal as we see a brief break in between larger-scale
waves. Kept the forecast dry despite a few models hinting at a few
rogue sprinkles flirting with our far west late. However, this
break in forcing is small enough in time/space that the odds of
having "crystal clear" skies all night (most favorable for
widespread frost formation) is not high everywhere either. For
one thing, if the low stratus from today is stubborn to vacate our
northern/eastern counties by sunset, it`s not out of the question
that it could try to linger into the night. Meanwhile, while much
of the central/especially west should start out the night
clear/mostly clear, as gradual invasion of at least "some" mid-
high clouds from the west will likely occur as the night wears on.
On the positive side for efficient cooling/frost formation, a
surface high pressure ridge will result in northerly breezes
markedly decreasing post-sunset and especially post-midnight, with
generally light/variable breezes most areas late in the night.
All things considered, and with at least part of the night likely
to feature some decent clearing, did not stray too far from the
latest guidance and our previous forecast on low temps, with most
areas dropping into the 30-35 range, and far northern areas most
favored to perhaps see upper 20s. Some folks in these colder
forecasted areas may wonder why we`ve opted for a Frost Advisory
versus a Freeze Warning, and the short answer is that we tend to
reserve formal Freeze Warnings for widespread "hard freezes" where
places are expected to drop below 28 degrees for several hours,
and tonight is not looking like one of these setups. In summary
and in closing, despite the possible caveats for realizing full
cooling and frost potential tonight, there was enough concern to
collaborate with neighboring offices and pull the trigger on the
Frost Advisory given that at least parts of the CWA will likely
see some.

.LONG TERM...(Thursday daytime through Tuesday)
Issued at 356 AM CDT Wed Apr 26 2017

At the start of the long term period Thursday morning, models are in
pretty good agreement showing zonal to low amplitude northwesterly
flow in place in the upper levels, thanks to low pressure spinning
over northern MN and another disturbance moving out of the Rockies.
Expecting a frosty start to the day, as a surface ridge axis and
light winds look to be in place. Through the daytime hours, the
first of a couple of system to affect the area will be moving in
from the west, increasing precipitation chances. The better chances
are expected to move in during the afternoon hours, and with it
being a slower mover, chances are lingering all the way into Friday.
Models aren`t in too bad of agreement, showing the main mid level
frontogenetical band sliding from SSW to NNE, with just some minor
timing differences. Through ~12Z Friday, the better chances look to
be roughly along/south of a LXN/HJH line, moving north of that line
after 12Z Friday. Through the afternoon/evening hours on Friday,
that northward shifting precipitation looks to also be waning. Hard
to say at this point how much of a lull (and the areal coverage of
it) there may be, but the CWA will be sitting between the departing
disturbances phasing in with low pressure over Ontario and deepening
low pressure over portions of CO/NM. No notable change to the well
below normal highs expected both Thur/Fri. Normal is roughly mid
60s, forecast highs for Thur are in the lower/mid 50s, with Fri in
the upper 40s to lower 50s. Precipitation type looks to be a cold
rain, the colder temps that would support a mix of snow are expected
to remain west of the CWA, and instability/thunder chances remain
south.

Looking to the upcoming weekend (late Fri night through Sun
evening), focus remains on the above mentioned deepening upper level
low pressure system off to our WSW. At 12Z Saturday, models are in
pretty good agreement showing the upper low located over the western
CO/NM border. While larger scale lift is on the increase, at least
through 12Z Saturday, there are questions with just how far north
into the CWA any precipitation is reaching. Latest runs showing
lower chances north of the NE/KS state line. That question continues
through the daytime hours Saturday, as the main low has only shifted
closer to the OK/TX panhandle area. There is more notable
differences between models with the areal coverage, the ECMWF/GEM
are on the more liberal side with their QPF, the GFS struggles to
get to I-80. Model difference start spreading more Sat night into
Sun, a little with timing but more so with the path of the upper
low. At 12Z Sun, the ECMWF/GEM have the low centered roughly over
western portions of our CWA, the GFS is further south, along the
central KS/OK border. While both tracks bring good precip chances to
the CWA, the GFS is showing a greater impact over the eastern half
of the CWA and exiting out of the area quicker. The others affect
more of the area and are on the slower side.

Saturday morning and more so Sunday morning, colder air working in
with the system will bring the potential for at least a RA/SN mix
(if not a total switch over to SN). Saturday morning, this potential
is at this point confined to far northwestern portions of the area.
Sat night/Sun morning, at least a portion of every county in our CWA
has at least a RA/SN mix mention in the the forecast. Far western
portions may see a switch over to all snow for a period of time.
Daytime highs are forecast to reach into the 40s, but confidence in
forecast temps aren`t with the potential for ongoing precip during
the day. Too early at this point to get too caught up in
accumulations, etc...plenty of model details yet to be worked out.
It`s something to keep an eye on, as winds may also play a factor.
Sustained speeds of 15-25 MPH will be possible.

This system is expected to depart Sunday night, exact timing yet to
be determined. While much of the area is currently forecast to be
dry, confidence isn`t high, as models showing another disturbance
quick on the heels of the last. The pattern remains unsettled for
the first half of the work week, though a more mild air mass also
looks to build back in behind the weekend low, bringing highs back
into the 50s/60s.

&&

.AVIATION...(For the 06Z KGRI/KEAR TAFS through 06Z Wednesday night)
Issued at 105 AM CDT Wed Apr 26 2017

General overview:
MVFR ceilings will continue to dominate roughly the first half of
the period before lifting/dissipating to VFR levels. Winds will
remain quite breezy from the north-northwest through today before
easing up markedly this evening. Now some more element-specific
details for those interested:

Ceiling:
While confidence is pretty high that roughly these first 12 hours
should remain dominated by MVFR, as usual there are some question
marks regarding exactly when ceilings will lift to VFR and/or
scatter out, with some guidance suggesting as early as around 15Z
at KEAR and others stubbornly holding on to MVFR well into the
afternoon. So, as a current middle ground best-guess have aimed
for VFR arriving at KEAR at 17Z and KGRI at 19Z. Even if it is a
bit delayed from this, confidence is pretty high in considerable
clearing by early evening.

Surface winds:
Breezy north-northwest winds with sustained speeds generally
15-20kt and gusts 20-25kt will hold firm through late afternoon
before markedly declining under 10kt late in the period.


&&

.GID WATCHES/WARNINGS/ADVISORIES...
NE...Frost Advisory from midnight tonight to 9 AM CDT Thursday for
     NEZ039>041-046>049-060>064-072>077-082>087.

KS...Frost Advisory from midnight tonight to 9 AM CDT Thursday for
     KSZ005>007-017>019.

&&

$$

SHORT TERM...Pfannkuch
LONG TERM...ADP
AVIATION...Pfannkuch



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