345 
FXUS63 KOAX 200539
AFDOAX

Area Forecast Discussion
National Weather Service Omaha/Valley NE
1239 AM CDT Sun Jun 20 2021

...Updated Aviation Forecast Discussion...

.DISCUSSION...
Issued at 341 PM CDT Sat Jun 19 2021

Forecast Summary:

There's a good chance for thunderstorms overnight, some of which 
could be severe with large hail and damaging winds. A cold front 
sweeping through the region Sunday may spark a few more 
thunderstorms, but the main weather story then will be the cooler 
temperatures settling in to start the work week. A gradual warming
trend follows, with sporadic shower and thunder chances mid to 
late week.

Tonight and Sunday:

A relatively dry atmosphere was over eastern Nebraska and southwest 
Iowa this afternoon, north of a frontal boundary close to the 
Nebraska and Kansas border. Dew points were in the 50s north of the 
front as easterly winds reinforce the dry air. But lower 70s dew 
points were not far away, in northern Kansas and far southeast 
Nebraska. At the mid levels, high pressure continued to be shunted 
into the Southwest as stronger westerlies covered the Northern 
Rockies and Northern Plains. A few embedded ripples were triggering 
some elevated showers and thunderstorms in north central Nebraska 
and northwest Kansas. 

Expect the showers/storms in north central Nebraska to continue to 
drift through northeast Nebraska through the afternoon. Then the 
main show begins to set up as a more potent shortwave emerges 
from WY/CO region this evening. Strong theta-e advection ahead of 
this deepening wave will overspread central then eastern Nebraska 
and western Iowa later this evening and overnight, essentially 
dragging northern Kansas frontal boundary northward with it. High 
instability now south of our area will surge into southeast 
Nebraska with advance of warm front, with elevated instability 
north of frontal zone increasing in kind across the rest of our 
area. MUCAPE greater than 2500 J/kg is likely across much of our 
CWA overnight, along with precipitable water values approaching 
200 percent of normal. 

Large hail will be possible with any of the elevated storms north of 
warm front, especially late this evening and overnight. There is a 
concern surface-based storms could pose a tornado threat through the 
late evening and early overnight period as increasing and veering 
low level jet overspreads frontal zone. Large, looping hodographs 
are noted by several models in southeast Nebraska near and south of 
warm front. Shear vectors from a west-southwest direction near 40kt 
are expected to intersect line of convection nearly perpendicularly, 
suggesting supercell structures and potential for meso-vortices 
along QLCS. Will have to see how things play out as our thinking is 
a little more aggressive in regards to severe and tornado potential 
than SPC outlook.

Main batch of storms should be entering eastern Nebraska at late 
evening, then progress east through western Iowa by 12Z. There is
some potential for storm development ahead of the main line, 
which could affect evening activities, but those would be spotty. 

Storms should be exiting well to the east Sunday morning before 
initial cold front following shortwave dives southeast. Despite 
northwest winds, dew points are forecast to remain relatively high 
(60s), adding fuel for potential convection along cold front. Most 
model output suggests areas southeast of Interstate 80 have the 
better shot at afternoon convection, and if it does occur, some 
severe risk will come with it. 

Sunday night through Saturday:

Secondary surge of cooler air is forecast to overspread the region 
Sunday night, significantly cooling temperatures and dew points for 
Monday. Highs in the 70s are still looking likely then. 

Beyond Monday, mid level flow pattern will favor a gradual warming 
trend as heights rise with eastern trough lifting northeast and 
Southwest ridge expanding eastward through the Southern Plains. 
There is fairly decent agreement between longer-range models through 
mid week, but solutions diverge thereafter. ECMWF suggests southwest 
flow aloft much of the period before dropping a trough into the 
Plains Friday. GFS represents a faster entry of that trough into the 
region. Suffice it to say, with either solution we should see 
occasional shower/thunder chances as shortwaves ride through the 
flow. The main difference between these solutions is in 
temperatures, especially Friday and Saturday when there is a wide 
spread in model output. GFS is the coolest with highs in the 70s 
while ECMWF is warmest with highs in the 90s. Our forecast is closer 
to the 75th percentile of all models, but confidence is low in this 
regard.

&&

.AVIATION...(For the 06Z TAFS through 06Z Sunday night)
Issued at 1240 AM CDT Sun Jun 20 2021

VFR conditions will prevail through the TAF period. 

Thunderstorms will work from southwest to northeast from 06Z to 
12Z. KOFK will likely miss the thunder but could see scattered 
rain. Widespread lightning with the bowing line of storms. Expect 
surface wind gusts near 40knots at times... with the occasional 
gust up to 60 knots. CB at FL040 with worst of storms. 

After 06Z, expect a lull in the action before a cold front sweeps
through and changes the winds from southerly to NNW. This will
happen between 15Z and 22Z from NW to SE. A few thunderstorms are
possible with that passage, but chances are too low for TAF
mention at this time. OMA with best chances with this second wave. 

&&

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

$$

DISCUSSION...Dergan
AVIATION...Nicolaisen