Area Forecast Discussion
Issued by NWS Des Moines, IA
FXUS63 KDMX 271004
AREA FORECAST DISCUSSION
National Weather Service Des Moines IA
404 AM CST Sun Nov 27 2016
.SHORT TERM.../Today through Tonight/
Issued at 404 AM CST Sun Nov 27 2016
Radiational cooling having big impacts across northern
Iowa. Models incorrectly proggd mid/high level clouds to help impede
fog development. However, those mid-level clouds over the Great
Lakes region now and clear skies have been in place. Temperatures
across northern Iowa have fallen to the mid 20s, leading to freezing
fog. With no apparati to lead to notable temp increases before
sunrise, expect freezing fog to remain a hazard through sunrise as
temperatures to remain below freezing.
This afternoon into tonight...
06Z Water vapor imagery picking up on an impulse of moisture
trekking across the Great Plains towards Iowa associated with a
shortwave that is expected to phase with a deep upper low located
near the Rockies along the Canada/U.S. border. Models have become
extremely well-converged in terms of their solutions...so have gone
with an overall consensus blend for timing. Accompanying sfc low to
be in Nebraska by 21z Sun and over eastern SD by around 06z Mon.
With the low well to our west, we are able to tap into sufficient
gulf moisture for precip via a 45kt LLJ. PWATs are over an inch,
which is high for this time of year. Even a few rumbles of thunder
are possible as 0-6km muCAPE values make it into the 300 j/kg range.
Soundings are very well-saturated in the low levels, suggesting a
very cloudy day at the very least...and cloud thicknesses deep
enough for sustained precip beyond drizzle.
With the expected cloud cover and precip, Sunday daytime
temperatures may be slightly overdone, so will need to monitor this.
As we remain in the warm/WAA sector overnight, diurnal temperature
range between Sunday and monday will be very low. In fact,
temperatures may increase slightly....leading to the possibility of
record high min temps.
.LONG TERM.../Monday through Saturday/
Issued at 404 AM CST Sun Nov 27 2016
The upper level low pressure will be centered over the eastern
Dakotas on Monday. The mid level dry slot will be over Iowa Monday
morning. Some uncertainty regarding how much stratus will be
lingering Monday morning and how deep. If the low stratus is still
intact, likely will have a few areas of drizzle. A boundary will
move through central Iowa Monday afternoon with a few showers
possible ahead of the front. The NAM solution is the most
aggressive with weak convective instability developing ahead of
the boundary and is an outlier at this time therefore will keep
the mention for thunder out.
The upper low will progress very slowly east across Minnesota on
Tuesday and Wednesday then become more progressive Wednesday night
and Thursday as it reaches the eastern Great Lakes. Conditions
will be cooler and mostly dry on Tuesday. Light precipitation
chances will arrive Wednesday as a more cyclonic flow develops
with the upper low beginning to transition further east.
Precipitation type will be a mix of rain and snow. Deep cold air
will exist above 2500 ft with precipitation type and melting
dependent on sfc wetbulb temperatures. With deep cold air aloft
need wetbulb temperatures near 34.5 F to ensure melting and a
transition to rain. This will be close on Wednesday but a
reinforcing push of cold air will aid the transition further
Wednesday night. Light accumulations of snow may occur over
northern Iowa and cannot discount accumulations over 1 inch.
The end of the forecast period will remain cool with highs in the
30s. The potential for very light snow or flurries will persist
through the period as northerly flow continues and periods of
stratus move through.
.AVIATION.../For the 00Z TAFS through 00Z Sunday evening/
Issued at 536 PM CST Sat Nov 26 2016
Mainly VFR conditions will prevail through mid-morning Sunday. The
exception is across northern Iowa, where we may see some fog
development once again overnight. An old, decayed surface trough
is draped across southern Minnesota where winds have gone calm
beneath clear skies. However, up toward MCW winds are still light
from the southwest and it remains to be seen whether either the
trough or associated fog will get as far south as MCW. By midday
tomorrow an approaching storm system will result in rapidly
expanding and lowering ceilings across the region, with showers
and possibly a few thunderstorms bringing IFR conditions by the
mid to late afternoon and evening. MVFR ceilings may arrive a few
hours before the rain, but IFR ceilings will be tied to it.
Dense Fog Advisory until 9 AM CST this morning for IAZ004>007-