Area Forecast Discussion
Issued by NWS Omaha/Valley, NE

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FXUS63 KOAX 200425

Area Forecast Discussion
National Weather Service Omaha/Valley NE
1125 PM CDT Tue Sep 19 2017

.SHORT TERM...(Tonight through Friday)
Issued at 329 PM CDT Tue Sep 19 2017

Midsummer-like conditions have returned to the area under strong
warm and moist air advection in the lower levels. Subjective 12Z
upper-air analysis indicates an upper-level trough digging into the
western US and ridging in the east, with an embedded shortwave over
IN/IL. 850mb dewpoints of 12C+ had pushed into NE/western IA by 12Z
on rather strong 850mb southerly winds. Surface lows at 20Z were
centered in central SD and northeast CO, with a cold front between.

Main forecast concern is whether storms will brush northeast NE this
evening and tonight, then chances again on Wednesday evening in
southwest Iowa. For tonight, 18Z soundings from both OMA and ABR
indicate substantial cap, reinforcing RAP mesoanalysis of the cap
persisting in NE. Where cap has eroded in central SD, convective
development is likely later this afternoon into this evening, with
convection persisting tonight. Should any development occur in or
move into northeast NE, abundant low-level and deep-layer shear
would support potential for storms to become severe, with large hail
and damaging winds the main risk, as well as a non-zero risk for
tornado development.

Cold front will progress through the area tonight and Wednesday,
bringing cooler temperatures. Additionally, shower/thunderstorm
development is possible in southwest Iowa mainly in the evening as
the front encounters a more unstable atmosphere toward the
southeast, but the risk for stronger storms is likely to stay
southeast and out of the area.  Warmer temperatures will return
quickly for Thursday and Friday under return flow, along with a
moistening low- to mid-level airmass under renewed warm/moist air

.LONG TERM...(Friday night through Tuesday)
Issued at 329 PM CDT Tue Sep 19 2017

Pattern shift continues to show itself for the weekend into next
week, with an upper-level low digging into the Rockies/western US
and ridging over the eastern US. A slow-moving front through the
Plains will provide focus for prolonged moderate to heavy rainfall.
Timing continues to slow in the model guidance, and have slowed
onset of precipitation further today. Have considerable uncertainty
regarding convective side of the precipitation. Convective
instability looks likely, especially in the earlier part of the
prolonged event, but widespread coverage of showers and storms in
September could quickly erode that instability and leave the rest of
the weekend mainly rainy. Have kept thunder in the forecast for now,
but think it is likely that later in the weekend and into next week,
the event would be more rain than thunder. Rainfall could persist
over several days, with moderate to heavy accumulations. Current
ground conditions could take some rain, but urban areas and at least
flashier creeks and streams could be susceptible to some flooding,
especially if rainfall rates are on the higher side. In short, it is
a potentially wet weekend into early week, but the details are still
unclear. At the very least, areas that are under clouds and rain
will be on the cool side, while those areas that stay ahead of the
rain Friday and Saturday will still be able to warm up above normal.


.AVIATION...(For the 06Z TAFS through 06Z Wednesday night)
Issued at 1124 PM CDT Tue Sep 19 2017

A surface cold front will drop into the TAF sites overnight
turning winds around to the northwest. Have continued mention of
LLWS for KOMA and KLNK overnight as 50-60kt low-level jet remains
in the area. Otherwise, VFR conditions will prevail.




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