Area Forecast Discussion Issued by NWS Omaha/Valley, NE
000
FXUS63 KOAX 130831
AFDOAX
Area Forecast Discussion
National Weather Service Omaha/Valley NE
331 AM CDT Sat Apr 13 2024
.KEY MESSAGES...
- Very high fire danger is expected today, especially over
southeast Nebraska and far southwest Iowa.
- Thunderstorms late Monday into Monday night may become severe.
Currently there is about a 20 percent chance for severe
weather within 25 miles of any location in eastern Nebraska or
southwest Iowa late Monday.
- Another round of strong to severe storms is possible Tuesday
afternoon, with the best chance currently over southeast
Nebraska and southwest Iowa.
&&
.DISCUSSION...
Issued at 202 AM CDT Sat Apr 13 2024
Looking at the big picture in water vapor this morning, we are
under broad upper level ridging with evidence of a weak short
wave trough entering Colorado and a deeper trough off of the
northern California coast. A lee surface low was over east
central Colorado with additional lee low pressure over eastern
Montana. With the surface high ridge over the Mississippi
valley, the low level jet response over the Plains has been
broad and pretty strong with 50-60 kts at 500 m. Surface winds
are much lighter, 10 mph or less out of the south southeast, but
enough to keep temperatures a bit warmer as most sites are in
the mid 40s to low 50s while a few calm locations in Iowa have
crept into the 30s.
Today`s weather focus is on nailing down the intricacies of the
temperature, relative humidity, and wind gust forecasts...
specifically as they apply to fire weather and potential for red
flag conditions. The strength of the existing low level jet is
the first concern as it is quite a bit stronger than model
guidance would have suggested. So, those first wind gusts of the
day may be on the stronger side, but RH won`t be particularly
low yet. RH will become quite low by the afternoon hours though
as mixing becomes deeper into the very dry layer that exists
above the boundary layer. Wind speeds in this layer will
decrease by mid day, but increase late, and occasional gusts of
25-30 mph over far southeast NE into extreme southwest IA
appear possible especially late afternoon into early evening.
This overlap of occasional gusts to around 30 mph with very warm
temperatures and RH dropping into the 16-20 percent range brings
us right up to the edge of Red Flag Warning criteria. There are
two limiting factors. One is the ongoing greenup which has
provided some improvement to fuel moisture in the past week, and
the other is the presence of what should be some decent cirrus
cloud cover by late in the day which just may be enough to limit
mixing a bit. So at this point, have opted not to issue an RFW
but it`s really close and any expectation of more frequent 30+
mph wind gusts could prompt a morning issuance. Other than fire
weather, Saturday will be warm and breezy, but winds near/north
of I-80 will be a bit lighter through the day and generally less
than 20 mph.
The weak short wave over CO moves east overnight and brings a
weak cold front through the area. It really acts more like a
simple wind shift for Sunday as highs will only be about 5
degrees cooler in the upper 70s to low 80s. The airmass will be
quite dry once again, but winds will be lighter out of the
northeast on Sunday, and should preclude any significant fire
weather concerns.
Monday into Tuesday brings an interesting shift in the weather
as that low currently off the west coast will move into the
Rockies. Models are fairly consistent in indicating that the
main low will be preceded by weaker bursts of short wave energy,
and the center of the system itself seems likely to pause as
embedded waves cycle around the main low before deepening and
ejecting with a slight negative tilt on Tuesday. Expect 60
degree surface dewpoints to move into the region by Monday and
linger into Tuesday. A weak warm front will lift into the local
area Monday into Monday night while the dryline and Pacific
front look to hold well to our west. The warm frontal zone
should provide a zone of enhanced convergence Monday evening and
overnight to support convective initiation. The details are
fuzzy though as model soundings indicate the likelihood of a
fairly dry capping inversion on Monday night north of the warm
front which isn`t particularly favorable for convective
development, but there could still be a favorable zone across
the forecast area in that time frame. Farther south, there are
indications that surface based convection may be possible well
into the overnight hours. Whether surface based or elevated,
deep layer wind fields are sufficient for storm organization
with large hail and embedded severe wind gusts...and any
overnight surface based storms may support a tornado threat as
well. As of now, would plan for a bit of severe storm activity
in the forecast area with the best chance in the late evening to
early morning hours, but a non-zero chance of some development
even in mid-afternoon along that drifting warm front convergence
zone.
Tuesday`s scenario holds great interest for severe weather
potential as well. The system and surface low will move west to
east into southeast NE and western IA. All model guidance has
continued to trend slower with this eastward progression, slow
enough that the effective dryline will cross the southern half
of the forecast area between roughly noon and 4 PM. Forcing will
be quite strong along this boundary and it seems to be a
favorable setup for a string of potentially severe storms to
develop. Forecast wind profiles are messy, but the
synoptic/mesoscale setup is one that would favor some supercell
organization with large hail and a few tornadoes possible...and
is certainly a time frame to watch closely.
Rain moves out by Wednesday morning with cooler temperatures
filtering in. Northern stream trough energy over the Canada/ND
border region absorbs the Tuesday system with deep westerly flow
overhead. So there will be some cold advection, but without much
of a northerly component it is unlikely to be very cold. There
is good model agreement in a fast moving weak short wave moving
west to east across the area late Wednesday, likely interacting
with the existing temperature gradient to enhance frontogenesis
and develop precipitation. There is a bit of a signal for snow
late Wednesday night, but a vast majority of ensemble guidance
is warm enough for rain. So for now this is just something to
keep in mind but it seems that it will be hard to bring in cold
enough air to snow in this deep flow pattern. Cooler
temperatures with highs in the 50s persist through the end of
the forecast period.
&&
.AVIATION /06Z TAFS THROUGH 06Z SUNDAY/...
Issued at 1102 PM CDT Fri Apr 12 2024
VFR conditions are expected to last through the period, with
much lighter winds than before out of the south-southeast at
less than 10 kts. A nearby fire has wafted smoke near the KOMA
terminal, and has resulted in brief visibility reductions. Low-
level wind shear is expected to affect KOFK and KLNK with 40 kt
winds at FL014 starting at around 08z through 14z. Tomorrow
some mid and high clouds are expected to waft through during the
late morning into the afternoon with wind gusts around 20 kts
out of the south expected to return until around or just after
sundown. &&
.OAX WATCHES/WARNINGS/ADVISORIES...
NE...None.
IA...None.
&&
.OAX WATCHES/WARNINGS/ADVISORIES...
NE...None.
IA...None.
&&
$$
DISCUSSION...Barjenbruch
AVIATION...Petersen