Scientific Forecaster Discussion
fxus63 kdmx 201135
Area forecast discussion
National Weather Service Des Moines Iowa
535 am CST Mon Nov 20 2017
Short term.../today through tonight/
issued at 334 am CST Mon Nov 20 2017
Southwest winds have remained relatively steady in central and
southern Iowa overnight with temperatures holding in the mid to
upper 30s. Farther north, winds are a bit lighter and have seen
temperatures drop a bit farther with readings around 30. Overall
however, temperatures will be starting relatively warm for this time
of year with thermal ridging building into the state. Will see
decent southwest flow through the day but likely won't realize how
warm it could potentially be given an inversion around 3kft. In
addition, it appears the broad cirrus shield in the upper Midwest
and plains will spread across the state today which will put some
limit on insolation as well. Still, temperatures will be well above
normal with mid to upper 50s in most locations.
The southwest flow will persist for much of the night prior to a
cold front entering the state around midnight. Temperatures will
remain relatively warm in this modest southwest flow until the front
arrives late tonight and cold advection increases. Temperatures in
the far north should drop relatively quickly late tonight with
strong northwest winds developing aided by the strong cold
advection. However, the cold air has little chance of making it
farther south by the end of the period with overnight lows remaining
relatively warm for this time of year. With moisture remaining
limited, there is little threat of precipitation with the frontal
passage despite some increased forcing.
Long term.../Tuesday through Sunday/
issued at 334 am CST Mon Nov 20 2017
Bottom line up front...
forecast confidence remains high with a precipitation-free week
ahead that will feature cooler temperatures Tuesday and
Wednesday, followed by record to near-record high temperatures for
Friday. Strong northwesterly winds on Tuesday may have slight
Tuesday through Thursday...
09z Mon water vapor imagery showing nearly stationary upper low
from the past few days over northern Manitoba slowly pushing into
the Hudson Bay. Meanwhile, a shortwave is starting to take shape
over northern Alberta. For the past few days, models have rapidly
pushed this shortwave to the east across Canada and have it
phasing with the now Hudson Bay upper low. Iowa from this will be
the accompanying longwave trough digging down to the Ozarks,
bringing down strong cold air advection to Iowa. In fact 850mb temps fall from
the +5c to +9c range at 06z Tue, to the -5c to -9c range by 21z
Tue. With stratocu looking likely, will be difficult for
temperatures to rise much, if at all on Tuesday.
More importantly will be potential for gusty northwest winds on Tuesday.
Tightened pressure gradient of around 3mb/125 km to arrive at
15z. Concurrently, 3-hr pressure change of +3mb and low-level
subsidence in place. Forecast soundings pretty conclusive across
model spectrum of transport layer winds around 25kt to 35kt
across most of the dmx County Warning Area. Given all of this, have bumped up
winds, especially gusts, several kts vs guidance Tuesday.
Interesting plot Twist is forecast soundings, in particular kmcw
staying mixed through 00z. If this pans out, going forecast will
be around 10 kts too light. With travel up Tuesday afternoon and
even, extra caution is advised if traveling, especially for high-
profile vehicles traveling on southeast-NE oriented roads.
By 12z Wed, a 1030mb sfc high quickly makes its way into western
Missouri from the NW, kicking the high winds out of Iowa. By 18z,
this high is slated to be over central Missouri, ending the strong
cold air advection and gradually bringing return flow and warm air advection back into Iowa late
Wednesday into Thursday...
Friday and beyond...
by 12z Fri, warm air advection will have ushered 850mb temps of +11c to +14c
into Iowa. For those who missed the weekend's programming, the
past two morning afds have highlighted that the European model (ecmwf) has had this
trend in the forecast for this time since its 240-hr mark on the
12z Wed model run. The GFS has been slowly playing catch up. Thus,
have leaned towards the European model (ecmwf) again, and have bumped up
temperatures several degrees vs guidance. Guidance is starting to
close The Gap some... the past two 00z runs have been around 5 to
10 degrees too cold. Today's 00z guidance was only around 5
degrees too cold. Will likely see another iteration of increased
temperatures tomorrow morning. Trend also continues for being to
cold for Sat am, and possibly even Sat PM.
Max high Fri pm: des moines: 67 (2011), waterloo: 62 (2001)
high min Sat am: Des Moines and waterloo: 45 (2011)
This upcoming weekend may be a repeat of Tuesday and Wednesday,
with a shortwave racing across Canada and the accompanying
longwave trough digging into the Ozarks, cooling temperatures back
into the 30s and 40s. Similar to Friday, early next week hints at
the potential of another thermal ridge crashing down over Iowa,
with 850 temps potentially in the +10c to +14c range again...
obviously a lot of time between now and next Monday though.
Aviation.../for the 12z tafs through 12z Tuesday morning/
issued at 534 am CST Mon Nov 20 2017
Aviation weather will remain generally good through the entire
forecast period with widespread VFR conditions. Visibilities will
be unrestricted for the duration with any ceilings from high
cirrus clouds. Surface winds will be the most problematic issue
with gusty southwest winds by this afternoon. The winds will
diminish this evening and shift to the northwest behind a cold
front which will advance southeast through the state late tonight.