Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 220947
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
347 AM CST Thu Nov 22 2018

.SHORT TERM.../Today through Tonight/
Issued at 346 AM CST Thu Nov 22 2018

Thanksgiving will be generally quiet as a deep-layer ridge moves
over the region, combining with increasing south southeasterly
surface winds to promote modest warm air advection in Iowa. What
little remains of the snowpack in southern Iowa will melt as
temperatures rise to around 50 or a bit higher, while in our
northeastern counties temperatures will be held in check in the
upper 30s or lower 40s, still resulting in some snow melt.
Complicating the temperature forecast is the deck of stratus
currently developing rapidly over the northern half or so of our
forecast area. There is uncertainty as to how much this stratus deck
will erode or move out during the day, however given the strength of
the near-surface flow by this afternoon, it is likely that the low
clouds will either advect up into Minnesota or be mixed out. There
is also some question as to whether light fog/stratus will develop
in the north/northeast again tonight, but again, given surface winds
of 10-15 knots any fog should be light. Finally, after midnight
tonight a low pressure trough will rapidly approach from the west
and spread thickening clouds across Iowa, but it appears the
precipitation associated with this system will hold off until after
12Z Friday and the forecast for late tonight remains dry at this
time.

.LONG TERM.../Friday through Wednesday/
Issued at 346 AM CST Thu Nov 22 2018

Forecast challenges remain the same as previous forecasts with
precipitation arriving on Friday, a winter storm impacting the
Midwest Saturday night into Sunday night and another round of cold
early next week. A short wave trough will arrive across the area by
early Friday morning. Expect warm advection will be ongoing at most
locations by 12z Friday. Areas of rain will develop and become
widespread during the day as the precipitation lifts northeast
across the state. Expect temperatures to be above freezing at all
locations once precipitation begins. Some hints at limited
instability as the system passes by with low static stability and
near saturated adiabatic lapse rates above 500 mb for a short
period. Outside chance at localized lightning but at this point
confidence is low enough to exclude.

The upper level system that will impact the region late Saturday
into Sunday is currently over the Pacific west of British Columbia.
The system will arrive over the Pacific Northwest on Friday then
settling southeast into the Intermountain West on Saturday before
ejecting out into the Midwest. There remain a lot of variables at
play regarding the evolution of this system. Model solutions are
continuing to have problems resolving the upper low over
Saskatchewan and western Ontario during this timeframe. It is this
system that will have a significant impact on the track of the
Pacific system. At this point, have low confidence with the GFS
solution which is farther south with the Canadian upper low and has
transitioned to a faster open wave solution south. Prefer the ECMWF
solution at this time which features a closed upper low farther to
the northwest and is further supported by other deterministic
solutions despite the typical spatial and temporal differences. The
hazard threat remains focused over the southeast 1/2 to 2/3s of the
state with the potential for moderate snowfall in addition to strong
winds with some gusts over 40 mph possible. These conditions would
have significant travel impacts Sunday into Sunday night. Confidence
remains low to medium on the exact track of this system due to the
aforementioned variables. Confidence is increasing that there will
be a 6+ inch band of snow to the northwest of the low pressure
track with profiles favorable for a period of good dendritic growth.

Steep northerly flow will develop in the wake of the weekend system
and will help drive another round of cold air into the state. High
temperatures will not get out of the 20s on Monday then likely areas
across the north do not reach 20 on Tuesday. It is possible even
these temperatures are overdone should the weekend snow occur.

&&

.AVIATION.../For the 06Z TAFS through 06Z Thursday night/
Issued at 1025PM CST Wed Nov 21 2018

Primary challenge will be returning restrictions with overall low
confidence given trends so far this evening. Stratus clouds have
cleared the central Iowa terminals at 430z; however, expect some
redevelopment of clouds and denser fog with restrictions
returning mainly to the northern terminals. Will also monitor for
possible fog development over southern Iowa tonight that would
require amendments. While conditions should return to VFR during
Thanksgiving Day, visibility restrictions may return along with
low level wind shear developing Thursday evening.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

SHORT TERM...Lee
LONG TERM...Donavon
AVIATION...Ansorge


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