Scientific Forecaster Discussion

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Area forecast discussion 
National Weather Service quadrant cities Iowa Illinois
1141 PM CDT Tuesday Apr 15 2014

Synopsis...
issued at 337 PM CDT Tuesday Apr 15 2014

Cold Canadian airmass under a ridge axis from eastern Texas to WI was
producing an early March-like day over the forecast area with
afternoon temperatures in the 30s to lower 40s under near full
sunshine. Upper air analysis from this morning indicated a broad northwest
flow from the West Coast to a longwave trough from the Great Lakes
to the central Gulf states. A shortwave was evident in the northwest flow
over Idaho and western Montana. Low pressure was developing in response
over eastern Montana. This shortwave and associated low will migrate
east-southeast passing through southern Minnesota or northern Iowa Wednesday night. The
tight pressure gradient and strong low level jet well in advance
will create windy and somewhat warmer conditions Wednesday.

&&

Short term...(this evening through wednesday)
issued at 337 PM CDT Tuesday Apr 15 2014

Primary challenge is the windy conditions developing Wednesday and
the uncertainties in critical low level moisture profiles.

Tonight...clear skies and very dry air will provide another chilly
night...with lows in the 20s north to around 30 south during the
first half of the night before surface winds begin to respond to
the approaching low in the plains. With dewpoints only in the
teens...will be counting on the high clouds advancing from the plains
to suppress the radiational cooling and prevent lows from dropping
even further.

Wednesday...south winds should increase to 20 to 30 miles per hour by middle
morning with gusts into the 40s possible by afternoon...especially
if model indicated 850 mb winds as high as 50 to 60 kts are
correct. A Wind Advisory may be needed. Still some uncertainty and
the anticipated thickening middle and high clouds may also limit the
mixing and intensity of wind gusts...so will hold off on headlines
at this time. The warm advection...however...is not nearly as
strong as the past few storm systems and mixing to 850 mb to 800
mb would only yield highs in the upper 50s to lower 60s. Another
large uncertainty...critical for fire weather...is the amount of
low level moisture. Forecast models have had a poor handle on
moisture and mixing in similar scenarios this Spring...and
currently show dewpoints in the 30s for tomorrow...whereas the
upstream airmass in the plains has again mixed dewpoints in the
teens and 20s. If this were to happen...relative humidity values may be low
enough to warrant a red flag warning...as discussed in the fire
weather discussion.

Long term...(wednesday night through tuesday)
issued at 337 PM CDT Tuesday Apr 15 2014

Forecast focus on near to below normal temperatures through the
weekend then trending above normal early next week.

Wednesday night and Thursday...occluding/weakening storm system
tracking into Wisconsin will keep the bulk of the rain and snow
north of the dvn County Warning Area. However...locations near Highway 20 will have
a slight chance Wednesday evening. On Thursday a weak cold front
will arrive with another chance of showers especially in our southeast County Warning Area.
Moisture will be limited as strong high pressure ridge will extend
from the East Coast into the Gulf of Mexico. Maximum temperatures on
Thursday will range from the upper 40s northwest to the middle 50s southeast.

Thursday night through Friday night...high pressure will be building
into the region with highs in the 50s and lows in the 30s.

This weekend...storm system will be tracking across southern Canada
with a slow moving cold front pushing across the Midwest. Modified
western Gulf moisture will be pulled northward ahead of the
front...allowing for light to possibly moderate rain sometime on
Sunday into Sunday night. Milder temperatures can be expected with
highs pushing into the middle 50s to middle 60s.

Early next week...a strong storm system will be organizing in the
northern rockies which will pull above normal temperatures into the
dvn County Warning Area. Highs should push well into the 60s to lower 70s.

Haase

&&

Aviation...(for the 06z tafs through 06z Wednesday night)
issued at 1137 PM CDT Tuesday Apr 15 2014

Strong south winds will develop by Wednesday morning as low
pressure moves from South Dakota into north central Iowa by late
Wednesday afternoon. South winds at 20 to 25 kts gusting to around
30kts are expected 12z-15z with speeds increasing to around 25kts
with gusts around 35kts by 17z and continuing through the afternoon.
There remains a concern about the potential for low level wind
shear late tonight as south winds around 2kft increase to 40 to
50kts. Since the increase in speed with height will be gradual and
winds will be unidirectional through this layer...the threat for
significant shear still looks marginal. Wednesday evening the low
will lift northeast into Wisconsin...dragging a cold front through
central Iowa. A few showers just ahead of the front may make it
into the kcid vicinity just before 06z. Winds will also be
decreasing ahead of the front...but should remain gusty.

&&

Fire weather...
issued at 337 PM CDT Tuesday Apr 15 2014

Strong south winds of 25 to 35 miles per hour are expected over northeast
Missouri and eastern Iowa from Wednesday morning through afternoon.
Relative humidity values will drop into the 30s and possibly as
low as the teens and 20s by afternoon. If these lower values are
reached...it would lead to very high to extreme grassland fire
danger index values. Fire danger will be elevated and possibly
high over dry grasses and cured vegetation.

If forecast confidence increases and suggests widespread relative
humidity values will indeed drop into the teens and 20s
Wednesday...a red flag warning may be needed for portions of
eastern Iowa and northeast MO.

&&

Dvn watches/warnings/advisories...
Iowa...none.
Illinois...none.
MO...none.
&&

$$

Synopsis...11
short term...11
long term...haase
aviation...dlf
fire weather...11