Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Sioux Falls South Dakota
1051 PM CDT Monday Apr 20 2015
Short term...(this evening through tuesday)
issued at 308 PM CDT Monday Apr 20 2015
A corridor of very dry air has mixed down this afternoon across the
County Warning Area...with early afternoon dew points as low as 4 degrees. Large
upper trough over the western Great Lakes continues to deepen...with
winds gusting above 40 knots.
For the overnight hours...winds will gradually weaken and
decouple...but remain rather mixy overnight. Temperatures will fall
near the 30-32 degree mark briefly...but not anticipating any
Tuesday presents another interesting forecasting challenge. Models
suggesting a spoke of energy rotating around the upper
trough...bringing a weak backdoor cold front through the area early
in the day. NAM/Gem only model suggesting enough moisture to produce
light quantitative precipitation forecast...however sub-cloud layer remains very dry...and anything
that does fall may just be sprinkles.
Mechanical mixing will again drop dew points lower than models
suggest in the afternoon hours...with relative humidity values near 20 degrees
again. Strato-cumulus may clip the eastern zones in the afternoon and
have lowered temperatures a couple degrees.
Long term...(tuesday night through monday)
issued at 308 PM CDT Monday Apr 20 2015
Focus in the medium range continues to be the dry conditions leading
to an elevated fire weather danger...and the sub-freezing
temperatures at night. Clear skies and decreasing winds will lead
to good radiative conditions across the area both Tuesday and
Wednesday night. With dew points tumbling well into the teens
during the day...temperatures have a long ways to fall in the dry
air. Winds will stay up on Buffalo Ridge more Tuesday night than
Wednesday night...but with 925 hpa temperatures near 0...expect lows to
mainly be in the 20s. Am not anticipating frost given the dry
nature of the atmosphere...but lows in the 20s could impact early
budding plant across the region. Certainly the potential for
widespread freeze headlight Tuesday night...and potentially one
north of I-90 Wednesday night.
Surface ridge shifts east and southerly winds begin to develop on
Thursday. Across the east...may hang on to more of a northwesterly
flow as momentum drawn from aloft overcomes weak surface gradient.
Little moisture return initially with southerly flow...which will
lead to elevated fire weather conditions continuing. The saving
Grace is that short grasses are starting to green up. With lack of
moisture...have cut back probability of precipitation with wave moving across the Central
Plains late in the week. Bulk of the forcing remains south of the
forecast area anyway...but with little moisture to work with...it
will likely be tough to get measurable precipitation.
Wave progressing eastward late in the weekend from the Pacific
northwest will be the only true shot of moisture over the next week.
Left guidance probability of precipitation in the forecast for Sunday into Monday for now.
Aviation...(for the 06z tafs through 06z Tuesday night)
issued at 1047 PM CDT Monday Apr 20 2015
VFR will prevail for the rest of tonight and Tuesday. There will
be some middle level cloudiness moving from northwest to southeast
through the day on Tuesday...but thats about all. There will be
wind again however as another reinforcing shot of cool air moves
southward. Expect northwest winds to frequently gust to around 30
knots or a little more by afternoon. But it should not be as windy
as Monday was as the mixed layer is not as deep.
Red flag conditions continue over half of the area this afternoon as
dew points have mixed into the single digits and winds have gusted
as high as 44 knots. These conditions will continue through sunset
before modest improvement this evening. The only thing saving
portions of Iowa from rfw warning conditions is saturdays
rainfall...and a bit more of a Greenup.
Tuesday presents an interesting challenge for fire weather. A weak
surface front will pass through the area early in the day...however we
will begin to mix quickly behind it. Again...have lowered dew
points below guidance and resultant relative humidity values are again below
critical levels. Winds will mix down weaker than today for
sustained winds...but frequent gusts above 30 knots are possible.
We will certainly be near or at red flag criteria in many areas and
perhaps further southeast into western portions of Iowa. Strong
winds and low humidity today should finally neutralize any rainfall
that fell near the Missouri River on Saturday. Later shifts will
need to watch for the potential of a red flag warning for
Tuesday...which falls in line with Storm Prediction Center d2 fire weather outlook.
Conditions will likely be near red flag again Wednesday afternoon
across southwest Minnesota and across portions of northwest Iowa.
Relative humidity values are expected to fall into the 15 to 20 percent range
across the area...keeping the fire danger in the very high to
While winds will be relatively light on Thursday...minimum relative
humidity values will be in the 15 to 30 percent range.