Area Forecast Discussion
Issued by NWS Des Moines, IA
FXUS63 KDMX 280452
Area Forecast Discussion
National Weather Service Des Moines IA
1152 PM CDT Thu Apr 27 2017
...Update for 06z Aviation Discussion...
Issued at 645 PM CDT Thu Apr 27 2017
Closely watching clouds on visible/IR imagery that are trying to
clear out of north central Iowa. Also, on the WV channel am
seeing moisture returning over southern Iowa in the way of high
and mid-level clouds. Concern as discussed in the short term
below is that clouds will clear, temperatures will drop quickly,
and more widespread frost will develop. While patchy frost is in
the grids, may need to consider an advisory later this evening if
clearing trend continues. As a climatology note, normal date of
last frost over northern Iowa ranges from May 6 through May 11.
Otherwise, forecast is on track this evening.
.SHORT TERM.../Tonight through Friday/
Issued at 343 PM CDT Thu Apr 27 2017
Immediate concerns for tonight revolve around the clearing of the
stratus and temps. Soundings are showing drier air trying to come
down and undercut the stratus which has pretty much cleared the
Southwest third or so of the state. The drier air tried to work
further North as evidenced by satellite showing the clouds
becoming more cellular in nature but they have since filled back
in so confidence in the Northeast clearing out is becoming more
suspect. I believe there will be a window of clearing into the mid
evening but mid clouds from the next system are already working
into the far Southwest and I believe the stratus will have a
tougher time clearing the Northeast. As a result, I have cooled
temps down based on where the coldest temps reside at 3 PM.
Obviously if the clouds clear out of the Northeast tonight then
the bottom will drop out on the temps but for now I have lower
30`s going with upper 20`s touching the Northwest where clearing
is a little better. Across the South where temps got a chance to
climb some I have lows around 40.
Frost is of some concern as well across the North...and especially
Northwest. Tonight has a couple things going for frost such as
colder temps to start with and lighter winds however, clouds should
hang in and limit frost potential. I did put in patchy frost
thinking that if there is a brief period in an overall cloudy sky
where we break up then frost could form.
Beginning late tonight but especially Friday the upper low over the
Plains begins to pivot Northeastward. A surface low over the
Southern Plains will develop a warm front that will shift Northward.
Moisture will advect Northward and far Southern Iowa will quickly
saturate with that moisture spreading North through the morning. The
best forcing will pivot NE but remain mainly West of the forecast
area (with the exception of the West/West central part of our
forecast area) thus rainfall will overspread central Iowa through
the morning then slowly diminish through the mid to late afternoon.
With the clouds, rain and a Northeast wind at 10 to 20 mph, Friday
will be a dreary damp and cool day. I knocked highs back a couple
degrees with mainly 40`s except for the far South.
.LONG TERM.../Friday night through Thursday/
Issued at 343 PM CDT Thu Apr 27 2017
Rainy weekend and a potential for snow were the main forecast
concerns over the extended. Leaned toward a blend of the ECMWF/GFS
throughout the forecast period as they are in decent agreement
into early Monday.
Shortwave embedded within the southwest flow aloft ahead of a deep
trough digging into the Desert Southwest will track east of the
state tomorrow night. This will leave a brief break in the rain
until mid to late Saturday morning when the aforementioned deep
trough continues to dig and progress east-northeast across the
Southern Plains Saturday.
Decent isentropic upglide and moisture transport along the
290-300K surface increases and transitions across the forecast
area (south to north) Saturday afternoon into the evening. Rain
will develop during this time and be fairly persistent into Sunday
as the aforementioned deep trough slowly progresses across the
region. Another slug of theta-e advection and moderately strong QG
forcing moves northward through the forecast area Sunday
providing a decent period of moderate rain. Profiles are cold
enough and moisture is deep enough for saturation within the snow
growth region and thus leading to ice introduction across
northern Iowa. The main area, or at least the best potential for
snow is northwest portions of the forecast Sunday into Sunday
evening. Kept with mention of snow Sunday before colder air
filters into the state behind of the low pressure system turning
many of the profiles to all snow b/t 00-06z Monday across the
west. GFS does have some fairly strong omega just below the
dendritic layer Sunday night with the CAA. Although not a
bullseye, might be enough to cause for decent snowfall rates and
lead to a little snow accumulations. Also to note this will be
occurring during the night providing a better potential for snow
to accumulate. Lingering precipitation is expected on Monday with
strong CAA throughout the day. Lowered maximum temperatures and
increases wind speeds Sunday and Monday with the forecast area
remaining well inside the CAA regime of the low pressure system.
Tuesday into Thursday...High pressure looks to try to build into
the region Tuesday into early Wednesday and trended towards a drier
forecast. Temperatures are expected to return to near normal by
Tuesday and Wednesday.
.AVIATION.../For the 06Z TAFS through 06Z Friday night/
Issued at 1152 PM CDT Thu Apr 27 2017
Stratus clouds that were producing MVFR ceilings over the
northern terminals sites earlier this evening have largely moved
out of the area. A thin band of showers is moving toward KOTM and
have VCSH, but drier air may win out. Otherwise, VFR conditions
will prevail tonight as high and mid-level clouds are streaming
into southern Iowa. Ceilings and visibilities will lower from
south to north through the day Friday as rain moves into the
state. The MVFR ceilings and showers may come near KMCW. As the
showers end Friday afternoon, IFR conditions may develop over the