Area Forecast Discussion
Issued by NWS Omaha/Valley, NE

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000
FXUS63 KOAX 211700
AFDOAX

Area Forecast Discussion
National Weather Service Omaha/Valley NE
1200 PM CDT WED SEP 21 2016

.SHORT TERM...(today through Friday)
Issued at 330 AM CDT WED SEP 21 2016

Thunderstorm chances this afternoon through Thursday are the primary
forecast concerns this period. Warm temperatures to continue as well.

Upper flow pattern was beginning its change as strong westerlies
across the Northern Plains were progressing east as strong
low/trough dropped into the Pacific Northwest early Wednesday.
Southwest mid level flow will remain over the Central Plains through
tonight, then shortwave ridging will build into our region later
Thursday into Friday. Shortwave within southwest flow aloft is shown
currently by RAP analysis moving through eastern Colorado, heading
toward Nebraska and Iowa later today and tonight. Meanwhile quasi-
stationary surface front extended from northeast Colorado into
southeast South Dakota and western Wisconsin. This boundary will not
move a whole lot much of the day as upper flow parallels front,
waiting for surface low tied to upper shortwave to develop and move
from central Nebraska into northern Iowa this evening. Increased low
level convergence ahead of the low will help ignite storms this
afternoon, then southward drift of front will occur in its wake
helped along by convection.

Expect storms to take some time to get going later today as
inhibition is slowly eroded and where low level convergence is
maximized in northeast Nebraska and southeast South Dakota ahead of
surface low. Instability increases markedly from near 1500 j/kg of
MLCAPE in northeast Nebraska to closer to 2500 j/kg in northwest
Iowa as per GFS, so we may miss initial development northeast of our
area around 00Z. As the evening progresses, southward drift of
surface front behind low combined with increasing moisture transport
over that boundary as low level jet kicks up will bring our best
chance for convection. Appears the best conjunction of those
parameters will lie somewhere between Columbus and Norfolk,
northeast to between Wayne and Tekamah, and into Monona County Iowa.
Expect storms to fire somewhere in this zone by 03Z, then expand
east and west with time. Continued moisture feed into south and
southwest flank will lead to some propagation southward toward
Interstate 80 overnight, but storms south of the Interstate may be
hard to come by.

Certainly could see severe storms given degree of instability but
overall bulk shear is modest at best. Storms becoming more outflow
dominant and loaded with precipitation suggests damaging wind gusts
or microbursts are possible as well. Heavy rain is also likely given
high precipitable water values near 1.75 inches shown by GFS, and
potential for training storms.

Bulk of overnight activity will be pushing off to the east Thursday
morning. Overall mid level pattern of building shortwave ridge
across our area will support northward propagation of frontal
boundary, but will not clear our northern border until later
Thursday night. Thus redevelopment of convection appears likely
Thursday afternoon where models support 2000 j/kg of ML and SB CAPE
and Bulk Shear is near 30kt.

By Friday morning building heights and front lifting north should
leave most of eastern Nebraska and western Iowa under capping mid
level temperatures. Surface temperatures should reach the upper 80s
again most spots Friday afternoon. In fact much of eastern Nebraska
and western Iowa will average mid and upper 80s for highs each
afternoon today through Friday. While highs Tuesday reached the 90s
all areas, more cloud cover today will likely hold temps down a tad,
though areas along and south of Interstate 80 could top 90 again.
And Thursday afternoon northeast Nebraska will be north of
retreating front and may not reach 80 in some areas.

.LONG TERM...(Friday night through Tuesday)
Issued at 330 AM CDT WED SEP 21 2016

GFS and ECMWF are fairly consistent in showing upper low rotating
into the Northern Plains on Saturday with trough axis extending into
New Mexico. They are also fairly close in depicting plume of
tropical moisture streaming into our region Saturday then being
shoved east by Sunday evening. Those models then diverge quite a bit
beyond that, with GFS offering a deep closed cyclone over Wisconsin
solution by Tuesday, and ECMWF maintaining general trough over our
region. As far as sensible weather in concerned, we will be on the
cool side of the system with pockets of cold air aloft rotating
across the region. Some of these may spark some showers from time to
time, but the main weather story appears to be temperatures in the
60s or lower 70s for highs to start next week.

Leading up to that, we will see periods of showers and thunderstorms
Saturday into Sunday when moisture plume is on top of us. GFS again
is painting precipitable water values of 1.75-2.00 inches across our
area Saturday. Thus heavy rain remains a concern Saturday and
Saturday night before bulk of activity pushes off to the east.

&&

.AVIATION...(18Z TAFS for KOFK...KLNK and KOMA.)
Issued at 1159 AM CDT WED SEP 21 2016

Gusty southerly winds will diminish this afternoon, with mainly VFR
conditions expected through the day.  A few showers and
thunderstorms could fire this evening and continue into the
overnight hours, however confidence remains low in the exact
locations that will be impacted. Therefore, included PROB30 groups
for -TSRA from 03Z to 07Z at KOMA and KOFK, and 04Z to 08Z at
KLNK.

&&

.OAX Watches/Warnings/Advisories...
NE...None.
IA...None.
&&

$$

SHORT TERM...Dergan
LONG TERM...Dergan
AVIATION...KG



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