Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Des Moines Iowa
107 PM CDT Friday Aug 28 2015
..updated aviation discussion...
Short term /today/...
issued at 330 am CDT Friday Aug 28 2015
Precipitation trends will be The Straight forward forecast issue today with
some temperature rebound concerns as well. Upper level flow will continue
to amplify today with passage of middle level wave through the MO
valley. Both kinematic and thermodynamic forcing are strongly noted
with some right entrance region support from WI/Michigan speed maximum and
DPVA ahead of eastern NE short wave mechanically. Fairly decent
baroclinic zone is also noted on 305k isent surface with NAM
depicting over 100 mb of lift across Iowa by 12z with 40-50kts of
flow. This results in very strong moisture transport and associated
convergence zones...which move slowly across Iowa reaching northestern
sections by early afternoon. Although the forcing is strong with
precipitable waters and warm cloud depths seasonally high suggesting high rainfall
efficiency...the lack of instability will hold things back somewhat
with convection only embedded within larger stratiform area.
Rainfall amounts will still exceed an inch in many locations however
with possibly over two inches in the central and eastern i80 to Highway
20 zones. Agree with wpc quantitative precipitation forecast assessment favoring this farther south
maximum axis. Do not feel rainfall rates will be sufficient to justify
any headlines at this point however.
Temperature rebound will be somewhat of a minor concern. Soundings suggest
there may be a little mixing boost in the warm sector for S central
and southeastern sections by afternoon behind the main precipitation shield...which
may also redevelop some showers or isolated storms...but clouds and
precipitation should offset too much of a rebound so stayed closer to
warmer MOS rather than what some raw temperatures suggest. Across the
north...clouds precipitation and an unfavorable more Ely wind should keep
any rebound to a minimum. However shear moisture addition from
precipitation and transport should nudge things up several degrees during
the heating of the day.
Long term /tonight through Thursday/...
issued at 330 am CDT Friday Aug 28 2015
Focus is mainly on temperatures throughout the period...and probability of precipitation
tonight. Surface low associated with the strong shortwave trough
dropping through the region today into tonight...will be centered
near Des Moines by 00z this evening. Much of the precipitation expected to
be on the warm side of the low and thus as the surface trough axis
shoves eastward through the County Warning Area tonight...expect precipitation chances to
diminish. Higher low level moisture noted with the models so do
expect an area of lingering low stratus across much of the County Warning Area into
early Saturday with the cloud deck slowly eroding/pushing east-southeast
through the day. With all the residual moisture around after the
rain...some clearing possible across the far west tonight...and
lighter winds setting in could see some fog development. Previous
grids had it mentioned across the western third of the County Warning Area...so
stuck with that areal coverage but did extend the mention into
Saturday morning. Otherwise large upper ridge across the central US
to amplify into build eastward into next week. This will bring
warming as the thermal ridge builds into the area and mainly dry
conditions. Models try to bring a few impulses moving through the
ridge Tuesday night and again Wednesday night...and may see an
isolated shower/thunderstorm. Better chances to the north and west.
Therefore did keep much of the period dry with just some slight
chance probability of precipitation across the far northwest Tuesday night.
850 mb temperatures cooler to begin the period in the teens celsius with
lingering cloud cover into Saturday especially across the east.
Therefore kept temperatures a bit cooler in the middle to upper 70s. As the
warmer airmass builds into the region and 850 mb temperatures expected to push
into the low to middle 20s celsius toward middle/late week have highs
warming into the middle to upper 80s and lows in the 60s. Dewpoints
also expected to increase back into the middle to upper 60s toward next
week making things feel a bit more humid.
issued at 105 PM CDT Friday Aug 28 2015
IFR to MVFR stratus will persist through the afternoon and into
the overnight hours. Mcw/alo have the best shot of IFR visible with respect to to
heavy rain this afternoon. Otherwise...expecting fog/stratus to
continue overnight into Saturday morning and have mention of IFR
conditions past 06z Saturday.