843 
FXUS63 KDMX 050825
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
325 AM CDT Wed May 5 2021

.DISCUSSION.../Today through Tuesday/
Issued at 325 AM CDT Wed May 5 2021

Forecast Highlights: 
-- Showers later today into tonight
-- Breezy Thursday, perhaps elevated fire weather
-- Rain and some storm chances, especially southern Iowa later 
   Saturday into Sunday

Details: GOES-East upper level water vapor imagery shows a shortwave 
trough moving through the Ohio Valley early this morning with a 
closed low circulation moving into southern Ontario as well. The 
closed low over southern Ontario will meander through this weekend 
into early next week close to Hudson Bay, which will keep the flow 
over our region generally beneath cyclonic through the end of this 
week. Within this flow, shortwave troughs will pass through, the 
first of which will be later today into tonight. Clouds associated 
with this wave are moving into eastern South Dakota and eastern 
Nebraska per GOES-East Nighttime Microphysics RGB. Clouds will 
increase and lower across central Iowa ahead of the shortwave and an 
associated weak surface low pressure. Weak theta-e advection along 
with modest QG convergence ahead of these features will aid in 
saturation. It will take a bit of time for saturation with forecast 
soundings showing a decent amount of dry air below 750mb. However, 
by later this afternoon expect showers to spread across northern and 
central Iowa. There remains very little in the way of instability 
and the only mention of thunder is in our western counties briefly 
late this afternoon and evening. Forecast soundings and plan view 
images show that any instability wanes quickly this evening. Showers 
will end tonight and clouds will depart as a cold front moves 
through the state. While temperatures will be a bit cooler on 
Thursday compared to today, winds from the northwest will turn brisk 
with wind gusts approaching 30 mph over northern Iowa toward and 
into the afternoon hours. The overturning of the boundary layer will 
also result in lower dewpoints being mixed down to the surface. 
Current forecast has values below the 25th percentile of available 
guidance from late morning into the afternoon hours. Current GFDI 
values would be in the high category, but if stronger winds, per the 
NAM, and lower dewpoints and associated relative humidity could 
create an elevated fire weather day over northern Iowa where fine 
fuels remain cured and have not greened up with the lack of 
rainfall. Finally, shallow cumulus may also be able to develop given 
the steep lapse rates in the low levels.

A potent PV anomaly will rotate around the Hudson Bay low and drop 
through Iowa Thursday night. The GFS deterministic is the most 
aggressive with bringing showers into central Iowa. Looking at it as 
well as the Canadian and European ensemble members, there are a few 
members that show the more aggressive solution. However, most keep 
the light QPF northeast of our area or the state and have kept our 
area dry during this time period.

The next chance for rain will come this weekend as ridging to the 
west of Iowa flattens in response to a trough moving over western 
North America. Surface low pressure will track through the Ozarks 
with a warm front remaining south of the state. While there will be 
rain north this front, the highest chances will be over southern 
Iowa with subsequent forecasts, if trends hold, being able to remove 
PoPs over northern Iowa. With Iowa on the cool side of the front, 
any thunderstorms will be elevated and have thunder mention only 
over southern Iowa. As the surface low moves into the Ohio Valley, 
this should bring an end to the rainfall on Sunday. The overall 
evolution of the mid-level pattern later this weekend into next week 
has quite a bit of divergence. WPC cluster analysis shows 
deterministic and ensemble members have a better handle on the 
Hudson Bay low, but have greater uncertainty with the western North 
America trough and whether a closed low develops. Initial guidance 
regarding rain chances next week were left as is given this 
uncertainty. 

&&

.AVIATION.../For the 06Z TAFS through 06Z Wednesday night/
Issued at 1115 PM CDT Tue May 4 2021

VFR conditions are expected at all terminals tonight through early
tomorrow with generally light and variable winds. A weak weather
system will produce increasing clouds and a chance for a few 
showers or thunderstorms late tomorrow afternoon into tomorrow 
evening, with the greatest coverage expected across northern and 
northeastern Iowa. Although there is some timing uncertainty, it 
looks as though MVFR CIGS will develop behind the cold front 
tomorrow night across the north. 

&&

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

$$

DISCUSSION...Ansorge
AVIATION...Fowle