Scientific Forecaster Discussion
fxus63 koax 151117
Area forecast discussion
National Weather Service Omaha/valley NE
517 am CST Fri Nov 15 2019
..updated aviation forecast discussion...
issued at 257 am CST Fri Nov 15 2019
Zonal flow in the plains today will be interrupted by a strong
shortwave trough diving southeast Saturday night, then a following
minor wave Sunday night. Both of these impulses will bring a chance
for precipitation. By mid week, mid level ridging is expected in the
central Continental U.S. As a closed low begins to form near Southern
California. However that ridging looks to be short-lived as another
strong shortwave dives into the Great Lakes, driving a cold front
through our region and bringing potential precip and cooler
Today through saturday:
Surface ridge over our southeast County Warning Area earlier Thursday evening had
slipped southeast this morning, allowing south to southwest low
level flow to return to the region. That flow will increase through
the day into Saturday as low pressure deepens in the Lee of The
Rockies with approach of strong mid level wave. Plenty of sunshine
today along with favorable low level flow and warmer atmosphere
should push highs close to normal this afternoon. Less mixing with
lighter winds east of the Missouri River will likely keep temps
there near or below 50.
By Saturday afternoon, 850 temps gain another few degrees, topping
out near 10c. However an increase in cloud cover and weaker mixing
in the afternoon point to similar highs to this afternoon.
Saturday night through monday:
Strong mid level wave will swing through the region Saturday night,
with a trailing minor wave Sunday night and Monday. Upper forcing
along and behind attendant cold front should be adequate to squeeze
out light precipitation Saturday night, especially in our north,
despite a relatively dry atmosphere. Temperature profiles suggest
rain is most likely precip type most of the night before
temperatures cool enough for a mix with light snow. However precip
should be waning as that occurs, so only looking at a small area in
west central Iowa where some flakes could be seen.
A quick shot of warm advection ahead of next minor clipper system
should keep temperatures at or just above freezing most of Sunday
night. Thus another round of mainly light rain is expected, but a
mix with sleet or freezing rain could occur in northeast Nebraska
and west central Iowa a couple of hours either side of 12z. Am not
expecting big impacts given spotty and light quantitative precipitation forecast.
Tuesday through thursday:
Mid level heights rise across the region through Wednesday,
suggesting temperatures should remain near or slightly above normal.
However the combination of a strong shortwave diving into the Great
Lakes driving cold air south into the plains, and warm air surging
north ahead of a surface low ejecting through Kansas from developing
southwest upper low, will lead to strengthening baroclinicity across
eastern Nebraska and southwest Iowa Wednesday afternoon and evening.
Right entrance region of 110kt jet in the northern plains combined
with left exit region of 80kt jet nosing into eastern Kansas places
our area under increased forcing where baroclinicity is greatest.
Thus precipitation is a good bet beginning sometime Wednesday
afternoon or evening and continuing at least through Thursday
morning when forcing subsides. While initial precip will likely be
rain, a quick change to light snow is likely later Wednesday night
as cold air deepens, especially across our north. Models are not
very consistent in regards to areal extent of precipitation, with
GFS the most bearish with quantitative precipitation forecast. European model (ecmwf) shows a tenth or so of liquid
quantitative precipitation forecast which may translate to some accumulating snows. Will have to
wait another few days to really get a handle on precip types and
amounts. However there is consensus on colder temperatures by
Thursday as north winds dominate.
Aviation...(for the 12z tafs through 12z Saturday morning)
issued at 515 am CST Fri Nov 15 2019
VFR conditions are forecast through Saturday morning, with mainly
scattered high clouds expected. Low level wind shear is expected
to develop overnight as a southerly low level jet increases to
over 40kt at fl020.