978
FXUS63 KMPX 141148
AFDMPX

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
648 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 /12Z TAFS THROUGH 12Z THURSDAY/...
Issued at 634 AM CDT Wed Aug 14 2024

Initialized with VFR and light winds this morning, with only EAU
potentially dealing with patchy fog over the next hour or so. The
main focus of these TAFs remains the rain for this afternoon
into 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. Used -SHRA to indicate
some sporadic rain coverage ahead of the main swaths of rain,
transitioning to -RA mention. Higher confidence of IFR overnight
has allowed for its addition to all TAF sites (except for EAU at
this point but IFR conditions may well be added in the next
cycle). Rain chances will continue past the end of the current
TAFs with degraded conditions likely continuing into Thursday
morning. CB/TS cannot be ruled out at any given site but high
uncertainty with timing/placement prevents mention at this
point.

KMSP...Initialized with VFR conditions and light winds which
will persist through early afternoon. Conditions then
deteriorate late this afternoon onward as the rain shield moves
into eastern MN. Still a little bit of wiggle room as to the
start time but it looks to be in the 22z-01z timeframe at MSP so
started there with -SHRA before going to more sustained -RA
thereafter. Low confidence on CB/TS so have omitted such
mention at this point. Regardless, MVFR conditions at best will
persist through tonight, dropping to IFR around daybreak
Thursday morning.

/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