Area Forecast Discussion
Issued by NWS Omaha/Valley, NE

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000
FXUS63 KOAX 181802
AFDOAX

Area Forecast Discussion
National Weather Service Omaha/Valley NE
102 PM CDT Thu May 18 2017

.SHORT TERM...(Today through Saturday)
Issued at 345 AM CDT Thu May 18 2017

Forecast concerns in the short term will be exiting storm system
then attention turns to the next weather maker begins to arrive
late this afternoon and tonight with another round of storms
Friday afternoon and evening...potential for severe weather and
rain amounts and temperatures through Saturday...and if a flash
flood/flood watch is needed and where.

At 00Z...the H5 pattern had a compact low pressure system over
northwest Iowa and another larger closed low over southern Idaho.
These h5 lows were vertically stacked with the h7/h85 lows
underneath. The 07Z surface metars highlighted northwest winds at
15 to 30 mph and temperatures in the 40s and 50s with dewpoints in
the 40s and 50. The cold front had pushed as far south as DDC and
was approaching ICT in Kansas and was near MKC. The WSR-88D radar
imagery indicated mostly dry conditions across the CWA.

We have a short reprieve from the wet weather as the closed h5 low
pressure system over northern Wisconsin exits the area...and the
large h5 low pressure system over Utah moves into Colorado and
lingers there into Friday until shifting into western Nebraska and
the western Dakotas later Friday and Friday night.

A potent H3 90-110kt jet diving into the west coast bottoms
and heads into eastern CO and Kansas today. Upper level divergence
focuses over western Kansas...however moves into the forecast area
this evening and South Dakota and western Iowa by Saturday
morning. Ascent increases over western Kansas and Nebraska
today...and into the forecast area tonight.

Wrap-around clouds will linger the longest in northeast
Nebraska...with cooler highs 20 to 30 degrees below normal in the
40s/50s near the SD border. Elsewhere...temperatures should top
out in the 60s to low 70s.

Showers and thunderstorms will be ongoing for parts of WY/CO/wrn
Neb. today. This afternoon...attention turns to severe weather
development/outbreak for parts of Kansas and Oklahoma ahead of
the dryline in the vicinity of the front...where low level
convergence/deep layer shear is maximized in a very moist and
unstable environment. With conditions coming together...SPC has
placed a high risk for severe weather in the area.

Locally, the surface low approaches the forecast area however it
appear to be north of the warm front and at the present time far
enough north to remain in the low 2% probability for tornadoes
with 25mi-LNK to Nebraska City to the KS border. The 60s and 70s
dewpoints are forecast to remain in Kansas with lower and mid 50s
dewpoints toward the Kansas border. 0-6km Bulk shear increases to
80kts.

Mid clouds will increase today and there may be a few showers or
sprinkles that try to develop. The better lift arrives in the
00Z-03Z time frame, so that is when we have the showers/tstorms
increasing and becoming more widespread overnight.  Although the
surface features favor areas to the south of the forecast area for
severe weather...mid and upper level features will provide plenty
of lift north of the front into the forecast area and elevated
thunderstorms. Our freezing levels should be a little higher than
the past couple of days more in the 10-11.5k ft range compared to
less than 10K feet...but still a little on the easier side to get
hail.  SPC has a marginal to slight risk for severe weather for
areas along and south of I80 and these would be more in the way of
the potential for large hail. Locally heavy rainfall may
accompany the storms with some 1 to 2 inch rainfall amounts. Flash
flood guidance is in the 1 to 3 inch range. For now have general
amounts of .5 to 1.25 inches tonight. WPC has the forecast area in
a marginal risk for excessive rainfall.

As the closed low has one area of lift into SD and IA Friday...the
band of higher rain amounts will lift to that region with more
scattered and light precipitation Friday. Later Friday afternoon
into Friday night...the low level forcing...re-surges across the
Plains into southeast Nebraska and southwest Iowa with additional
.5 to 1.5 inch rains possible. For day 2, there is a marginal risk
for severe weather and this will depend on the surface low track.
In addition, the slight risk for excessive rainfall covers a
larger part of the forecast area and the moderate risk touches
FNB. PWAT increases to 1.5 inches over southeast Nebraska and
southwest Iowa by Fri evening.

May need a flash flood/flood watch due to the elevated rivers
and streams and these two combined rain events/favorable
antecedent conditions.

The dry slot moves in for Saturday with some lingering
precipitation with the closed low...otherwise look for below
normal temperatures in the 50s and 60s.

.LONG TERM...(Saturday night through Wednesday)
Issued at 345 AM CDT Thu May 18 2017

Cool and drier Saturday night and Sunday with highs in the 60s. A
trough in the northwest flow aloft will bring a few showers to the
area Monday through Tuesday.

&&

.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 101 PM CDT Thu May 18 2017

MVFR ceilings prevail at all three TAF sites. Per satellite,
cirrus shield overspreading Nebraksa will inhibit deep mixing
somewhat, so delayed transition to low VFR ceilings until
late afternoon. Even if ceilings rise into VFR category, duration
will be short-lived, as large scale dynamic/isentropic ascent
will begin in earnest by evening. CAM suite all depicting northern
extent of a MCS moving into LNK 00-02UTC, OMA 01-03 UTC and OFK
03-04 UTC. Timing of most intense convection still unclear, so
broadbrushed at all three sites. Ceilings will vary between MVFR
and VFR due to convection, and then settle into prolonged period
of MVFR. Update to TAFs likely, as convection timing a mode
will have impact on precip timing and ceilings.


&&

.OAX WATCHES/WARNINGS/ADVISORIES...
NE...None.
IA...None.
&&

$$

SHORT TERM...Zapotocny
LONG TERM...Zapotocny
AVIATION...Fortin


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