875
FXUS63 KDTX 171928
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
328 PM EDT Fri May 17 2024

.KEY MESSAGES...

- Pop-up thunderstorms increase in isolated to scattered fashion
this afternoon, although severe weather is not expected with this
activity.

- An Air Quality Alert is in effect for portions of Southeast
Michigan Saturday, including Metro Detroit.

- Very warm and muggy conditions will be in-place this weekend with
additional opportunities for thunderstorms Sunday and again early
next week.

&&

.DISCUSSION...

Partial clearing after morning showers has allowed for areas of
sufficient insolation which converts building instability to surface-
based CAPE. Diurnal convective development has initiated in isolated
to widely-scattered fashion with surface dewpoints near 60F. Severe
threat remains quite low given 0-6 km bulk shear values AOB 25
knots, therefore expect mostly short-lived pulse-like thunderstorms.
Latest CAMs suggest a slight increase in coverage with the arrival
of a low-amplitude shortwave aloft. Varying storm motions possible
as a weak low pressure system/triple-point stalls over Midland.
Multiple boundaries within the vicinity offer several additional
point-sources for lift, augmented by MUCAPEs of 750-1250 J/kg.
Steepest lapse rates should be located within the 500-400 mb level,
exceeding 8 C/km for part of the afternoon while the lower layer
settles closer to moist adiabatic. Some pea-sized hail and few gusts
to 40 mph are the main threats today. Storms wane after 9 PM as the
mid-levels warm slightly and surface-based activity is cut-off by
the development of a nocturnal inversion. A couple lingering showers
are possible early tonight before the surface pattern gets washed
out.

Weak ridging ensues Saturday with anticyclonic flow noted above 700
mb. Dewpoints remain elevated (near 60F), but forecast soundings
reveal thermodynamic profiles that are a bit too warm to facilitate
convective updrafts. High temperatures warm to near 80F for most
areas while light southeast winds draw slightly cooler air to the
coastal communities off the lakes. Clouds thin heading into the
afternoon hours as a subsidence inversion develops between 800 and
700 mb.

Flow veers southwesterly Sunday as a deep closed low drifts into
northwestern Ontario. An attendant cold front extending well south
of the surface reflection washes out over Lake Michigan midday as
the warm advection arm traces back to northern Texas. Locally,
temperatures top out well above normal as highs peak in the low to
mid 80s marked by 850 mb temps of 14-15C. Note that some model
variance remains between the ensemble and deterministic solutions as
it pertains to thermodynamics. Additional differences noted in QPF
magnitude/extent, but most solutions do agree that some convective
storms should arise during the PM hours. Dewpoints in the lower 60s
and steep low-level lapse-rates should activate surface parcels as
shortwave perturbations embedded within the periphery of the leading
height fall gradient arrive. Agreed with NBM PoP increase to Slight
Chance across most of the forecast area. Low-end potential exists
for additional storms Sunday night as a more robust CVA response
streams through the drying mid-levels.

An unsettled pattern continues early next week as the synoptic ridge
becomes stationary over The Appalachians and the northern stream jet
remains active just off to the west. Several shortwaves will eject
eastward out of stalled longwave trough Monday and Tuesday with
additional ThetaE intrusions. Strong to severe storms would be most
probable Tuesday as dewpoints peak in the mid 60s and 0-6 km bulk
shear approaches 30 knots. The upper level configuration unlocks
Wednesday as an energetic low drops into the Pacific Northwest from
the Gulf of Alaska triggering ridge amplification over the
Intermountain West. This kicks the jet eastward Wednesday along with
a cold front which could initiate some preceding convection locally,
depending on timing and southern stream wave interactions. Post-
frontal conditions should be noticeably cooler for the back half of
next week with a return to near-normal readings.

&&

.MARINE...

Scattered showers potentially producing thunderstorms will move over
Lake Huron and Lake Erie this afternoon as a weak low pressure
center and frontal boundary move across the region. Gentle southerly
winds over Lake Huron will back to become more easterly this
evening, decreasing down to about 5 knots. These calm winds and
moist conditions will lead to some patchy fog overnight. Fog over
the lake will hang around for a bit longer than over land, lasting
into the early afternoon. Benign and predominantly southerly winds
persist through the weekend with a weak cold front moving through on
Sunday. Potential for more unsettled conditions Monday night into
Tuesday as a stronger system moves into the area. This system will
bring moderate southerly winds as well as showers and thunderstorms.

&&

.PREV DISCUSSION...
Issued at 136 PM EDT Fri May 17 2024

AVIATION...

A stalled cold front over the region will excite a round of showers
and thunderstorms this afternoon during peak heating. A lack of flow
will result in scattered coverage with slow moving storms making it
challenging to time out at any site this afternoon. Highest
confidence is in the higher terrain where FNT and PTK reside with a
line of showers already initiating. Lower confidence over the 3
Detroit area sites as the activity will only slowly drift toward
them through the later afternoon. Opted to go with a tempo for each
location though VCTS was considered for the Detroit sites. Storms
have already initiated and chances will carry through 01Z Sat. Heavy
downpours and/or a highly localized gust to or in excess of 40 knots
will be possible with the strongest activity. Attention tonight
turns to fog chances as light winds and clearing skies lead to good
radiating conditions.

For DTW/D21 Convection...DTW should reside on the eastern edge of
much of the convection that is ongoing as of 17Z. Most of the
activity is expected to stay to the west though there is a chance
it could drift over the airspace later this afternoon. The best
window for a thunderstorm or two will occur between 21Z-01Z. Locally
heavy downpours, brief visibility restrictions, or a highly
localized wind gust to or in excess of 40 knots will be possible
with any stronger activity.

DTW THRESHOLD PROBABILITIES...

* Low for ceilings 5000 ft or less this afternoon outside of
  convection.

* Low for thunderstorms this afternoon and evening, between 18Z -
  01Z Sat.

* Low for ceilings and visibilities to drop below 200ft or 1/2SM
  tonight.


&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...None.
Lake Huron...None.
Lake St Clair...None.
Michigan waters of Lake Erie...None.
&&

$$

DISCUSSION...KGK
MARINE.......BC
AVIATION.....DRK


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