Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Des Moines Iowa
644 am CDT Tuesday Aug 4 2015
Short term /today/...
issued at 411 am CDT Tuesday Aug 4 2015
Currently have an area of surface high pressure situated across much of
the state...with an area of low pressure developing along the Front
Range of The Rockies. Have a frontal boundary aloft stretching from
central and western South Dakota southeastward through eastern NE...SW Iowa and into
northern MO. Southerly 850 mb flow and warm air advection in place with Theta-E advection
as well. This has allowed for a line of thunderstorms to develop
along the boundary aloft. This front is expected to lift slowly
north/northeastward today with continued chances for showers and
thunderstorms in its vicinity. As for temperatures have kept maximum
temperatures similar today to readings from Monday with a similar airmass
in place across much of the County Warning Area. Areas seeing the warm air advection will also have
more cloud cover and the precipitation chances so overall maybe only warmed
them a degree or two at most.
Long term /tonight through Monday/...
issued at 411 am CDT Tuesday Aug 4 2015
By around 12z Wednesday...a shortwave looks to ride near to the Iowa/MO
border and clip southern Iowa. A boundary attached to low pressure
north of Montana looks to provide additional support for precipitation in
the form of weak low level moisture convergence. Precipitable waters appear to be
in the 1.5 inch to 1.75 inch range as this system propagates
eastward. Not a lot of support aloft as no middle to upper level jets
present. Minimal cape values plus shear of less than 20 kts...
points towards no convective/severe threat. 850mb temperatures for Wednesday very
similar to Tuesday 850mb temperatures. Forecast soundings suggest lower cloud bases
and more cloud cover Wednesday...especially south of I-80. Therefore...
started with persistence and then lowered Wednesday maximum temperatures a few
Active weather then sets up for Thursday into Friday. Models in
excellent agreement with passing a surface low eastward from South
Dakota Thursday morning into western Wisconsin by Friday morning.
Models also bring an attendant cold front with this low through
northern Iowa. Timing will be key with this boundary passage. As of
now...it appears the boundary will not enter the dmx County Warning Area until btx
00z Friday and 06z Friday. Severe threat with this boundary passage certainly
appears legit. 0 to 6 km bulk shear values in the 40 to 45 knots range
with this system. 0 to 6 km MUCAPE values hit the 2000 j/kg mark.
For severe weather Mode...dcape values near 1400 j/kg. Timing of boundary
passage highly climatological favors wind event. Large hail also
possible...especially early on and to the far northwest.
High pressure will be moving in behind this front...making it into
western Iowa by 00z Sat. This high should keep Friday PM dry and may
bring the sun back out. Cold air advection comes in advance of this high as 850mb
temperatures should be around 16c to 18c at 12z Friday. It looks like this
high should pass quickly with warmer air following this high to our
west. As of now...it appears the dmx County Warning Area should have a strong
northerly component to the flow during daylight hours Friday. If
future model runs speed up this high and get return flow to our
western edge by afternoon...maximum temperatures will certainly need to be
bumped up...likely by 5 degrees at least.
For the weekend...strong upper level high pressure re-establishes
itself near the OK/Texas Panhandle. With an upper level low expected to
skit along the Canada/US border...Iowa should once again return to
westerly/northwest flow aloft. Models try to bring several shortwaves into
Iowa Saturday through the end of the forecast period. So have left broad-
brushed probability of precipitation in the extended. 850 mb temperatures should be around 16c to
20c during this time period. With clouds and precipitation very
possible...this suggests temperatures slightly below normal in the
low to middle 80s.
issued at 644 am CDT Tuesday Aug 4 2015
VFR conditions expected for the critical taf period. High clouds
expected across the area with a cirrus shield in place from storms
across NE/SW Iowa. Frontal boundary lifting north/northeastward
today...will allow for rain chances to push into the western taf
sites tonight and all taf sites just beyond the current forecast
period. Have increasing lower clouds through the period...mainly
in the low VFR range. Winds to be light and variable becoming more
southeasterly through the period.