821
FXUS63 KMPX 140805
AFDMPX

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
305 AM CDT Wed Aug 14 2024

.KEY MESSAGES...

- Widespread rain will expand from west to east today. Heaviest
  rain is expected late this afternoon into tonight when storm
  potential is greatest.

- Lingering showers are possible to start the weekend before we
  resume our dry and mild stretch of weather through next week.

&&

.DISCUSSION...
Issued at 258 AM CDT Wed Aug 14 2024

We`ve been talking about it, and it`s finally here. The long awaited
system after our stretch of quiet weather is on the doorstep of
southwestern MN. The main threats from this system will be prolonged
rainfall with heavy rates and localized flooding possible at times.
There is a marginal chance for severe weather, mainly as damaging
winds, but isolated large hail and a tornado or two cannot be ruled
out. Let`s break it down...

Timing and Coverage... Recent CAMS have started to come into better
agreement, with the general idea of light to moderate showery precip
blanketing southwest MN by 9-11am. Current radar over eastern
Nebraska suggests that both the NAMNest and HRRR may be slightly
slow (by an hour or so). The 00z HRW Fv3 seems to have the best
handle on timing and coverage so far, so I`ll be favoring that
in the discussion here. Showers and a few isolated non-severe
storms will look to move into eastern Minnesota by the mid-late
afternoon, and western WI by early evening. There continues to
be uncertainty with how far north the initial wave of precip
will progress. The Fv3 has widespread showers moving into
north- central MN during the afternoon, however many of the
other models keep precip along and south of a line extending
from Stevens Co, MN to Rusk Co, WI. Scattered to widespread
showers and storms will continue into Wednesday night as the
surface low develops and gradually shifts NE through Minnesota
and northwestern WI. Portions of western WI could wake up to
widespread showers Thursday morning, otherwise the bulk of the
rain should have moved well north and east of the area. Much
more isolated to scattered rain is expected on the backside of
the low during the day Thursday. Rain chances fall off quickly
by Friday AM, as the surface low finally makes its way east
further into the Great Lakes region.

Heavy Rain and Flooding Potential... Forecast soundings show a deep,
saturated warm layer extending from roughly 1-1.5kft to 12-13kft,
weak instability at best, and PWAT values approaching 2". These all
favor an environment for high rain rates and efficient rainfall
production. Storm total QPF values (over nearly 36 hours) continue
to stay around 0.75-1.5", with the highest amounts falling late this
afternoon into tonight when the storm potential is greatest.
The highest probabilities of exceeding 1.5" (roughly 40-60%)
land within a line extending from Sioux Falls, SD up through the
Twin Cities metro and all way NE to Rice Lake, WI.

Severe Potential... Continues to look low, BUT the best timing and
location looks to be over southwest MN tonight or southern MN
tomorrow morning. While there should be sufficient shear and
forcing, the biggest limiting factors will be the lack of
instability and the deep saturation throughout the atmosphere (lack
of dry air). Some models, such as the HRRR, entertain the idea of a
more organized line of convection developing somewhere over the
Dakotas that attempts to progress SE along a boundary of 500-1500
J/kg MUCAPE. It is tough to say how far east this line would
progress into MN since the CAPE values really start to decline
across the border, however if it holds together, there could be
marginally severe weather of any threat (wind, hail, tornado). A
northward extension of the LLJ overnight into Thursday could also be
enough to support isolated storms, mainly over southeastern MN.

Weekend and Beyond... This weekend should see a return to
pleasant conditions again, with weak N/NW winds, partly cloudy
skies, and highs in the low 80s.

Both the EPS and GEFS H500 height anomalies hint at a relatively
quiet upcoming week, with persistent ridging over the central CONUS.
A shortwave or two may be enough for a few tenths of rain mid-week,
but nothing too impactful seems to be on the horizon following our
system today/tomorrow.

&&

.AVIATION /06Z TAFS THROUGH 06Z THURSDAY/...
Issued at 1242 AM CDT Wed Aug 14 2024

Initialized with VFR and light winds tonight, but patchy fog has
already developed in western WI (specifically EAU). Fog may be
possible at other sites outside of MSP but confidence not high
enough to include at other sites at this point. Will continue to
monitor through the early morning hours for possible TAF
updates. The main focus of these TAFs remains the rain for
Wednesday evening through Thursday. Rain will develop from
southwest MN and progress eastward. As rain starts, a drop to
MVFR is expected and eventually some IFR is expected as well.
The coverage of IFR is still uncertain, but confidence has
increased such that IFR ceilings were added for western and
central MN sites. Rain chances will continue past the end of
the current TAFs with degraded conditions likely continuing into
Thursday morning.

KMSP...Initialized with VFR conditions and light winds which
will persist through early Wednesday afternoon. Conditions then
deteriorate late Wednesday afternoon onward as the rain shield
moves into eastern MN. Still a little bit of wiggle room as to
the start time so have maintained the PROB30 mention for a
potential of rainfall to move in prior to 00z but confidence
increases significantly from 00z onward that rain will move
across the MSP airspace. Low confidence on CB/TS so have omitted
such mention at this point. Regardless, MVFR conditions at best
will persist through Wednesday night, possibly dropping to IFR
by Thursday morning. Will see how the model guidance develops
such a scenario for later TAFs.

/OUTLOOK FOR KMSP/
FRI...VFR, chc MVFR/-SHRA. Wind NW 10-15 kts.
SAT...VFR. Wind NW 10 kts.
SUN...VFR. Wind NW 5-10 kts.

&&

.MPX WATCHES/WARNINGS/ADVISORIES...
MN...None.
WI...None.

&&

$$

DISCUSSION...BED
AVIATION...JPC