FXUS63 KOAX 192326

Area Forecast Discussion
National Weather Service Omaha/Valley NE
626 PM CDT Mon Aug 19 2019

.SHORT TERM...(Tonight through Tuesday AM)
Issued at 328 PM CDT Mon Aug 19 2019

Thunderstorm chances overnight will be the main forecast concern in
the near term. Short range models have come more into agreement
today that storms are likely to develop in far southeast South
Dakota and northwest Iowa around midnight. These storms will
likely develop into a MCS which tracks southeast through central
Iowa. A majority of the models are in agreement that storms
should stay in central Iowa, although a few models show storms
which may clip our area through western Iowa.

IF storms do develop in western Iowa, conditions are favorable for
severe storms. ML cape values overnight will remain around 4000
j/Kg. Additionally, the low level jet should push into our area
bringing about 40 knots of shear. With ample moisture, any storm
that does try to sneak into our area will likely be strong or
severe. Brief heavy rains will also be possible, with models showing
precipitable water values near 2 inches overnight. Flooding should
not be a concern though with storms moving quickly out of the area
by mid morning Tuesday.

The most likely scenario overnight is that storms will remain east
of our area. However, if storms develop further west, they could be
severe. The main threats would be large hail and damaging winds.

.LONG TERM...(Tuesday PM through Monday)
Issued at 328 PM CDT Mon Aug 19 2019

Tuesday Heat
Tuesday will be very hot, and has a decent chance to reach at
least heat advisory criteria especially over the southern half of
the forecast area. However, after collaboration with neighboring
forecast offices, at this time have opted to hold off on issuance
of any heat advisory products in this area. The primary reason
involves uncertainty surrounding the outflow boundary and
potential cloud cover that may influence much of the forecast area
as a large convective complex is likely to move across Iowa
tonight. Dewpoints are very likely to be in the middle to upper
70s especially in the southern half of the area by Tuesday
afternoon but there could be some lingering cloud cover and
slightly cooler air in place to limit the overall heat index. That
said, with dewpoints that high, it will only take a small
difference of just a few degrees in afternoon temperatures to make
a big difference in the eventual heat index, and in this decision
we're looking at trying to predict the difference between 91 and
94 degrees. At this time predicting closer to 91, but this
decision will continue to be evaluated this evening and tonight.

Tuesday Night Storms
The likely outflow boundary will also influence thunderstorm
potential for Tuesday night. Very steep mid level lapse rates and
extreme instability are likely to be in place by late afternoon
owing to the hot and very humid boundary layer. However, the base
of the EML is very warm and will be an effective cap for most if
not all of the daytime hours. Expect large scale subsidence across
the region for most of the day in the wake of tonight's short wave
trough passage, but any remnant outflow boundary will need to be
closely monitored given the likelihood of extreme potential
instability collocated with weak boundary layer convergence.

As the evening progresses, it appears that a weak short wave
trough will begin to influence eastern NE while simultaneously
inducing an increasing low level jet converging over the local
area. These two forcing mechanisms appear likely to combine with
the existing instability to initiate storms during the evening
hours with the greatest focus in east central or southeast
Nebraska. The most plausible scenario would be for an area of
strong to severe elevated convection to develop in this area (and
also into central NE) with a predominantly hail threat by late
evening or early morning...and then evolving into one or more
bowing features riding along the effective boundary with an
increasing threat for damaging winds. Also, with multiple rounds
of thunderstorms and some training possible within the very
unstable and very moist atmosphere, flooding will become an
increasing threat over time.

Wednesday - Wednesday Night Storms
Wednesday's forecast remains unsettled and will be highly
dependent upon how Tuesday night unfolds. There is a chance that
periods of weak large scale forcing will persist in the region and
continue to generate periods of convection. At this time, it is
very uncertain in terms of strong/severe storm potential, but as
multiple rounds of storms impact the region over time the flood
threat will remain and perhaps increase. Otherwise, temperatures
will be much cooler for Wednesday as a cold front will have moved
through the area and widespread cloud cover appears likely.

Thursday through Monday
Cooler temperatures with highs in the 70s and 80s will continue
through the long term period. The overall flow pattern appears
likely to be zonal with a few minor disturbances and potential for
a stronger trough very late in the forecast period. Thus, have
several small chances for thunderstorms, mainly Friday night and
Sunday night into Monday.


.AVIATION...(For the 00Z TAFS through 00Z Tuesday evening)
Issued at 625 PM CDT Mon Aug 19 2019

A band of elevated convection over portions of eastern NE as of 6
PM will continue east into IA this evening with little if any
precipitation reaching the ground. More robust thunderstorm
development is anticipated late this evening from southeast SD
into northwest IA. Latest model data indicate those storms should
remain to the east of KOMA. Otherwise, we expect prevailing VFR
conditions with some potential for brief periods of MVFR
visibilities due to fog toward daybreak on Tuesday at KOFK.
Southeast winds at around 10 kt will switch to southwest Tuesday
morning. The exception could be at KOMA where winds late tonight
into Tuesday morning could be more erratic due to outflow from
storms over IA. Confidence in specific timing of that occurrence
is presently too low to mention in the forecast.




LONG TERM...Barjenbruch
Back to Main Forecast Discussion Page