Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 270227

Area Forecast Discussion
National Weather Service Des Moines IA
927 PM CDT Sun Mar 26 2017

Issued at 927 PM CDT Sun Mar 26 2017

Getting light dz over the south with likelihood that it
will remain patchy prior to arrival of light rain showers over
northern Missouri. Have added to forecast/grids across the south.
Light visby restrictions...but foggy areas not expected.


.SHORT TERM.../Tonight through Monday/
Issued at 310 PM CDT Sun Mar 26 2017

The primary concerns into Monday will be precip trends with the
approach of the current Central/Southern Plains system, as well as
clouds and fog to a lesser extent. Little change is anticipated into
the evening with persistence the best forecast, plenty of clouds and
visibilities gradually lowering again late. Areas of fog will be
expected again into the early morning hours but likely not to the
extent seen this morning with no recent precip and light north winds
lowering dewpoints ever so slightly. With stratus and overcast
conditions again anticipated through the night, have gone with lows
< 5F below current obs and above even the warmest MOS values.

The attention will turn toward the MO Valley by midnight with precip
approaching from the south and west. Deep fairly coupled kinematic
and thermodynamic forcing should reach southern Iowa early Monday
morning. The theta-e advection contributions will mainly be driven
by increasing moisture rather than warm advection as the baroclinic
zone stays primarily to the south. MUCAPE return is nil and forecast
soundings show moist adiabatic profiles so the precip should mainly
be stratiform rather than convective. The system core should
approach the MS River by early afternoon with a fairly well defined
back edge of forcing exiting southeast sections about that time with
dry conditions by 21z or 00z.

.LONG TERM.../Monday night through Sunday/
Issued at 310 PM CDT Sun Mar 26 2017

Our sensible weather this week will be dominated by a large and
slow-moving low pressure system moving from the Four Corners
region on Tuesday, to the southern High Plains on Wednesday, then
over Missouri and Iowa late on Thursday. After a brief period of
weak ridging over Iowa on Monday night and Tuesday, which may
allow for some brief clearing, on Tuesday night a large slug of
deep moisture will move in from the southwest in advance of the
approaching cyclone. If we do not see some sunshine on Tuesday
then we will certainly not see it for a while, as Wednesday and
Thursday will once again be gray and wet within the large
precipitation shield of the aforementioned low pressure system. In
fact, across about the southern half of the forecast area
categorical POPs are forecast for about 36 hours. Any warm sector
will remain to our south and instability is little to nil so no
thunderstorms are anticipated. On the other hand temperatures will
generally be too warm for any winter precipitation, though there
is a slim chance that on the tail end of the system as the low is
finally departing, there could be a quick switchover to non-
accumulating snow across our north right before the precipitation
ends. All in all, a long- duration rain event is expected.
Rainfall rates and overall amounts should not be sufficient to
induce any hydrologic concerns, though it is possible a few larger
river basins will see some modest rises. The only other sensible
weather note during this period is the effect of the clouds and
rain on diurnal temperature trends, especially hindering daytime
warming. Have continued trend of nudging high temperatures
downward for Wednesday and Thursday to account for this, but given
temperature trends of today and yesterday, the forecast may still
be too warm on those days.

The large cyclone will finally move off to the east on Thursday
night, replaced by another brief period of weak ridging on Friday.
Unfortunately the pattern will then repeat itself to some extent,
with long-range models indicating the slow advance of another
large low pressure system from the Four Corners to the southern
High Plains next weekend. Presently this system appears weaker
than the one several days earlier, but it could make for even more
dreary and damp weather at times next weekend. For now the POPs at
that range are relatively low due to uncertainties in the track
and timing of the parent system, but this will be monitored for
updates in the coming days.


.AVIATION.../For the 00Z TAFS through 00Z Monday evening/
Issued at 632 PM CDT Sun Mar 26 2017

Main concern still IFR cigs...though improvement aft 18z expected
with exiting low and slight push of drier air from the northwest.
Overall first 12-15 hours in the period will remain poor
conditions with light rain and fog also more likely at KOTM and
brief period at KDSM. Late in the period...aft 20z...expecting
most sites to rise to above 2000 ft cigs. Overall fog issues this
period not likely to repeat last night dense fog over east
/southeast. Will evaluate cig and br trends again for 06z package.




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