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Area forecast discussion 
National Weather Service Des Moines Iowa
330 PM CDT Tuesday Mar 31 2015

Short term /tonight/...
issued at 330 PM CDT Tuesday Mar 31 2015

Weak high pressure over the state today will shift east overnight.
This will result in an increasing pressure gradient over northwest
Iowa and return flow. Temperatures will slowly rise with the
development of return flow especially over the northwest. No chance
for precipitation overnight with the low level jet remaining well to
the west and moisture return occurring after the overnight period
though some high level clouds may arrive late.

Long term /Wednesday through Tuesday/...
issued at 330 PM CDT Tuesday Mar 31 2015

There is a myriad of potential hazards from Wednesday into
Wednesday night as a low pressure area deepens over the northern
High Plains and pushes a cold front across Iowa. Ahead of the
front...south winds will increase substantially by late morning
Wednesday with most short range models predicting winds of roughly
around 35 to 40 knots at the top of the mixed layer by 21z. This
already suggests the possibility of a Wind Advisory...albeit
somewhat marginal...for much of the area. In addition...there have
been reports of multiple spreading fires across the forecast area
today and with the stronger winds tomorrow and very dry surface
fuels remaining am very concerned about fire weather conditions
for Wednesday particularly in the afternoon. Forecast minimum relative humidity
values are 25 to 35 percent across much of the area...but this may
even be too high as there is a possibility that we may be
overestimating surface dewpoints in the afternoon during a battle
between moisture advection and turbulent mixing of very dry air
aloft. Have opted to issue a Fire Weather Watch for nearly the
entire forecast area as detailed in the fire weather section
below. A red flag warning may very well end up being issued for
Wednesday...but if not...then a Wind Advisory will be a distinct
possibility.

Moving on...thunderstorms are expected to develop from eastern
Nebraska up into southwestern Minnesota late Wednesday afternoon
and evening. It does appear that by the time they reach our area
it will be near 00z and have delayed probability of precipitation accordingly.
Nevertheless...there is strong support from an upper level trough
as well as a developing low level jet for sustained convection
dropping southeast across all of Iowa on Wednesday night and probability of precipitation
are 100 percent in most of our area at various times. Moderate
instability...steep lapse rates aloft...and deep layer wind shear
suggest a threat of severe weather early in the event...mainly in
our northwest...which should be elevated with wind and hail the
primary threat. The severe weather risk will then gradually
diminish as the storms move southeast after sunset. The currently
Storm Prediction Center outlooks are stellar in its discussion of this scenario. At
this time have left severe weather wording out of the
grids/forecast as the threat is maximized west of our area...but
this will certainly bear watching tomorrow.

After lingering storms exit the area Thursday morning a surface
ridge will slide by...bringing cool and quiet weather for most of
Thursday and Thursday night. However...the cold front that moved
across Iowa earlier will remain stalled over central or northern
Missouri by early Friday morning...when an even deeper upper level
trough will swing across the region. For the most part moisture
will be lacking for precipitation to develop across our
area...however...in the south and southeast we could see some
overrunning showers or storms and have maintained low probability of precipitation in this
area as the trough moves overhead on Friday. The European model (ecmwf) remains the
furthest north with its solution for precipitation...but a southern
solution seems more likely. Cloud cover from this system will also
help to keep temperatures down...with highs of only around 50
across Iowa.

This weekend the 500 mb flow over Iowa will take on a flat nearly
zonal character...likely resulting in mostly dry weather with
slowly moderating temperatures. As a large western U.S. Trough
builds a surface low will develop in the Lee of The
Rockies...extending an inverted surface trough up over Iowa by
Sunday. As the large 500 mb trough subsequently moves eastward
this will set up a fairly wet and active pattern for early next
week. The details are murky at this time...but certainly periods
of thunderstorm chances are warranted.

&&

Aviation...31/18z
issued at 1237 PM CDT Tuesday Mar 31 2015

VFR conditions will remain in place throughout taf period. Winds
will remain around 8 to 12 kts today and diminish this evening.
Winds will be out of the south tomorrow and pick up significantlyby
late morning and early afternoon...upwards of 20 to 25kts and
gusts to 30kts or greater.

&&

Fire weather...
issued at 330 PM CDT Tuesday Mar 31 2015

Southerly winds will increase even further on Wednesday...likely
reaching speeds of 25 to 30 miles per hour with gusts to near 40 miles per hour at
times. Given the very dry surface fuels across the area and the
ease with which fires are spreading today...there will obviously
be a heightened fire danger on Wednesday. There is some question
as to whether or not red flag warning criteria will be technically
met...as the current forecast calls for min relative humidity values between 25
and 35 percent across most of the area. However...the forecast
GFDI values are extreme for nearly all of central Iowa and given
the antecedent conditions find it prudent to issue a large Fire
Weather Watch at this time...which will likely be either
transitioned to a red flag warning or a Wind Advisory later
tonight.

&&

Dmx watches/warnings/advisories...
Fire Weather Watch from Wednesday morning through Wednesday
evening for Adair-Appanoose-Black Hawk-Boone-Bremer-Butler-
Calhoun-Carroll-Cerro Gordo-Clarke-Dallas-Davis-Decatur-Emmet-
Franklin-Greene-Grundy-Guthrie-Hamilton-Hancock-Hardin-Humboldt-
Jasper-Kossuth-Lucas-Madison-Mahaska-Marion-Marshall-Monroe-Palo
Alto-Pocahontas-Polk-Poweshiek-Ringgold-story-Tama-Union-Wapello-
Warren-Wayne-Webster-Winnebago-Worth-Wright.

&&

$$

Short term...donavon
long term...Lee
aviation...Curtis
fire weather...Lee

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