274
FXUS63 KDMX 120242
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
942 PM CDT Thu Jul 11 2024

 ...Updated for the 00z Aviation Discussion...

.KEY MESSAGES...

- Dry and relatively pleasant weather through Friday.

- Growing warmer this weekend, especially by Sunday when heat
  index values will top 100 degrees in some areas. This may
  occur again on Monday, especially in central and southern
  Iowa.

- Low thunderstorm chances return to northeastern Iowa early
  Sunday morning and early Monday morning, but are more likely
  in Minnesota and Wisconsin.

&&

.DISCUSSION...
Issued at 219 PM CDT Thu Jul 11 2024

As expected, cumulus field has developed today but has remained
fairly shallow with much less instability than yesterday. The
old upper low continues to move away northeastward into the
Great Lakes, yielding to an overall pleasant day across Iowa
with temperatures currently in the upper 70s to lower 80s and
light winds. Meanwhile, a large heat dome continues to build
over the southwestern and western U.S., and will become the
dominant weather influencer for our region through the weekend
and into early next week.

A surface high pressure ridge currently draped across much of
Wisconsin, Minnesota, Iowa, and northern Illinois will slowly
retreat eastward tonight and Saturday. This will allow low-level
winds to come around to southeast across Iowa by Friday morning,
then south on Saturday. Meanwhile, the nocturnal low-level jet
will return Friday night and we will thereafter see a steady
warm air/moisture advection regime over the weekend, concurrent
with the expanding influence of the aforementioned
western/southwestern heat dome. Initially, on Friday night this
set-up will result in some elevated instability however we will
still be well within the low-level subsidence regime of the
departing high which will help to squelch any convective
initiation. Then on Saturday and Sunday warmer temperatures
aloft should keep us capped off and dry for the most part,
however, this is a classic Ring of Fire scenario in which
shortwave impulses traveling over the top of the western high
trigger diurnally driven thunderstorms which then round the top
and turn southeastward around the periphery of the dome. Most
model solutions are predicting two or more rounds of such
thunderstorms over the weekend, however, initiation should be
well to our north over the Dakotas/Minnesota and at this time it
is uncertain whether the convection will be able to make it as
far south/southeast as our forecast area. If there is any such
threat then it would be likeliest in northeastern Iowa during
the early morning hours of Sunday and Monday.

Aside from some possible thunderstorm hazards in our northeast,
the bigger and more widespread sensible weather impacts from
this regime will be very hot and humid conditions spreading into
the region. Saturday will become warm, but the most oppressive
heat is set for Sunday, and Monday at least in some areas.
Ensemble temperature forecasts for Sunday have ticked up a bit
today, but there are still questions as to the mitigating
factors of evapotranspiration and limited mixing depth due to
capping. As previously discussed however, these factors are
somewhat offsetting in terms of the resulting Heat Index values,
which are likely to near 100 in the south and west on Saturday,
then approach 105 at times Sunday and may warrant a Heat
Advisory issuance in the next day or two. By Monday, however, a
large low pressure gyre will have formed over Hudson Bay, with
several troughs rounding its base and helping to push the
southwestern U.S. dome a bit farther south. This will allow a
frontal boundary to sag southward through Iowa sometime Monday
afternoon or night. Oppressive heat will thus continue on
Monday for areas that remain south of that boundary for most of
the afternoon, but there may be some relief in the north
depending on frontal timing. The initial frontal passage may be
relatively dry due to a stout cap in place during the day
Monday, however, the boundary will likely slow or stall
somewhere near the Iowa/Missouri border and on Monday night
active with somewhat more widespread convection as the low-level
jet feeds up into it. From Tuesday through Wednesday a large
surface high pressure area will then build southward from
Minnesota and the Dakotas into Iowa, gradually pushing the old
frontal boundary and any remaining associated rain/storms
southward into Missouri. This should bring relief in the form of
seasonally mild and dry weather to our area by the middle of
next week.

&&

.AVIATION /00Z TAFS THROUGH 00Z SATURDAY/...
Issued at 942 PM CDT Thu Jul 11 2024

VFR conditions are expected to prevail through the TAF period.
Patchy light fog is possible, mainly in southern Iowa, Friday
morning however recent data suggests this should not bring
conditions out of VFR so have kept from TAFs at this time.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

DISCUSSION...Lee
AVIATION...Hagenhoff