528
FXUS63 KMPX 141720
AFDMPX

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
1220 PM 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 /18Z TAFS THROUGH 18Z THURSDAY/...
Issued at 1205 PM CDT Wed Aug 14 2024

Most sites will initialize at VFR outside of SW MN sites (KRWF).
The main concern this period will be the rain this afternoon
through Thursday morning. Rain showers will gradually move
NE from SW MN into the Twin Cities and W WI by this evening. A
heavier, steady rain will begin after sunset. This will bring
the onset of low MVFR and IFR ceilings across the MN TAF sites
and WI sites Thursday morning. Continued -SHRA usage for
spottier coverage this afternoon and Tuesday morning and -RA for
steadier rain overnight. IFR is likely at all sites (outside of
EAU until later Tuesday AM) tonight. Lower confidence for any
TSRA tonight so I`ve opted to leave it out of the TAFs. We`ll
see rain begin to taper off Tuesday morning with a gradual
improvement of cig/vsbys by the end of the TAF period.

KMSP... VFR through mid-afternoon before lower VFR and MVFR cigs
settle in at MSP. Confidence remains high for rain to begin
later today and steadier, heavier rain tonight into Thursday
morning. Exact timing may vary by an hour or two either way of
the 18z MSP TAF given some discrepancies in the guidance. Start
time may trend from 22z to 23z-00z for instance. Low confidence
on TS/CB through 18z Thursday but opted for a PROB30 for
-TSRA/CB after 20z Thursday afternoon.

/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...BPH