Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Des Moines Iowa
336 am CDT Friday Jul 31 2015
Short term /today/...
issued at 336 am CDT Friday Jul 31 2015
Couple of challenges in forecast for today. First...subjective surface
analysis shows weak trough from northeast Iowa to near koma at 06z
with secondary weak cool front stretching from near kdlh south-southwest to
near St cloud and Aberdeen...both moving southeast with time
today. Weak upper level support across Nebraska combined with h850
moisture plume has unsettled airmass over northeast Nebraska/far
western Iowa with a small but increasing cluster of thunderstorms as
modest warm air advection at h850 and moisture plume from
southwest US toward eastern Nebraska interact with h850 upslope.
Hrrr model continues to develop the area south southeast with time
through 12z weakening it as it does. Middle to high level blowoff
will continue to advect downstream over central/southern Iowa
through middle morning. For now...will keep trends of weakening
convection/-shra over far southwest through 12z...and cloud deck
through most of day but drifting south with time. Whats left of
weak cool front will end up near the Iowa MO border by 00z with some
additional isolated thunder coverage over the far south by late
afternoon. In reality...no cooling aloft during the day so highs
once again push into the middle 80s north to the upper 80s near 90
from i80 south with warmer heat island effects accounting for the
near 90 mark today. Storm Prediction Center has raised severe threat to marginal
southwest quarter from south of i80/west of i35. Perhaps as front
settles toward the border this evening return flow from 00-06z may
provide enough lift to support some isolated hailers and wind. Wind
fields aloft fairly weak so most of the support should be thermal
instability through middle evening.
Long term /tonight through Thursday/...
issued at 336 am CDT Friday Jul 31 2015
Next severe weather and heavy rain potential will be Saturday
night into early Sunday. Then...an active weather pattern returns
throughout next week with several shortwaves riding the northwest
flow aloft combined with a boundary stalling across the state.
Models are in decent agreement through the weekend before the
bigger discrepancies show up between solutions Sunday night.
Tonight...weak Theta-E advection and isentropic lift provide
enough forcing to bring a few elevated showers and thunderstorms
across far southern Iowa late tonight into Saturday morning.
Introduced probability of precipitation past 06z Saturday but kept quantitative precipitation forecast amounts low at this time
due to the isolated nature of the convection.
Saturday into Sunday...a stronger shortwave brings the potential
for severe weather Saturday late afternoon into the evening. Deep
layer shear is favorable to sustained thunderstorms...but low
level shear remains weak and thus low confidence with any
tornadic activity Saturday night. Main threat appears to hail and
damaging winds with decent instability and middle-level lapse rates
in the 7.5-8c/km range. Heavy rain also a threat with models
depicting strong moisture transport into the state and warm layer
cloud depths range from 3500-3800 meters. Precipitable water
values in the 1.5-1.8 inch range and corfidi vectors somewhat
conducive to training storms...so certainly cannot rule out a few
locally heavy rain producers Saturday night. System moves out by
Sunday morning with the European model (ecmwf) slightly slower than the GFS but
looks to be dry across much of the County Warning Area by 18z Sunday at the
Monday through Thursday...front stalls across Iowa from west-
northwest to east-southeast by Monday and remains planted over the
state into at least Tuesday and likely into Wednesday. Depending
on which model...timing of the shortwaves to ride the upper level
ridge will be problematic. However...confident enough to increase
probability of precipitation Monday night as both European model (ecmwf)/GFS are at least in somewhat
agreement with widespread convection developing along the stalled
front as the low level jet increases. Very deep moisture...high precipitable
water values and near 4000 meters of warm layer cloud depths
Monday night should provide another round of heavy rain.
Additional shortwaves continue to move through the region from
Tuesday night into Thursday but again low confidence with timing
and location of these shortwaves and thus have small chance probability of precipitation
going during this time frame.
issued at 1126 PM CDT Thursday Jul 30 2015
Unless conditions change soon...expect VFR conditions across tafs
and no thunder with eastern NE convection staying SW of sites.
Convection seems to have cycled down and then back out of nowhere
recently however...so confidence in that has decreased somewhat.
Otherwise expect nothing more than scattered middle clouds and generally
light winds eventually veering to northwesterly during Friday peak heating