437
FXUS63 KGRR 131515
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
1115 AM EDT Mon May 13 2024

.KEY MESSAGES...

- Shower and Thunderstorm Chances Today into Tuesday

- Drying Out Midweek

- Periodic Risk for Rain Thursday Night into the Weekend

&&

.UPDATE...
Issued at 1115 AM EDT Mon May 13 2024

No big changes needed to the forecast for today. Skies have
cleared out quite a bit across the southern 2/3rds of the area,
with mid clouds holding on across the northern 1/3rd. The clouds
across the North are near a cold front that is trying to drop
through the area, but is slowing down.

The moisture in the lower levels is almost non-existent so far.
We are starting to see some sprinkles showing up at Cadillac and
Houghton Lake with some lower clouds starting to show up. This
will continue to expand across the North as better moisture starts
coming in.

Across the South, we do expect a few pop up showers and
eventually storms to develop. MU CAPEs are forecast to increase to
500-1000 J/kg this afternoon to support the thunder threat.
Severe weather threat is quite low with weak updrafts per forecast
soundings due to a lack of low level moisture, and deep layer
shear is very weak around 15 knots or so.

The better chance for widespread rain showers and some embedded
thunder will be later this evening and tonight as better
broadscale forcing coming in ahead of the upper trough interacts
with better moisture and the front across the South.

&&

.DISCUSSION...
Issued at 244 AM EDT Mon May 13 2024

- Shower and Thunderstorm Chances Today into Tuesday

A few lingering light showers this morning to exit to the east
prior to daybreak with a dry start expected this morning. Stalled
out front near I-96 to provide the focus mechanism for increasing
shower and thunderstorm potential mid to late this afternoon.
Severe weather is not expected.

Coverage of showers/storms should be near I-96 and south with
northern areas remaining mostly dry. Initial shower/storm
development will be more scattered in coverage before a transition
toward a more widespread steady rain occurs late tonight into
early Tuesday. Some locations could see over an inch of rain over
the next 24 hours or so. High pressure slowly building in will
begin to dry things out Tuesday afternoon.

Prior to onset of showers later today, temperatures will warm up
nicely with mid to upper 70s for most with a non zero chance of a
few locations topping out at or above 80 degrees.

- Drying Out Midweek

Modest mid level height rises and dry surface to 850 mb
northeasterly flow is expected for the midweek period beginning
Tuesday night and lasting through Wednesday. Low and mid level flow
veers southwest by Thursday out ahead of the next system slated to
impact the region by week`s end. But the majority of Thursday is
likely to remain dry as well at this point.

- Periodic Risk for Rain Thursday Night into the Weekend

Currently, the most likely period for rain is Thursday night into
Friday. Uncertainty exists regarding the synoptic pattern for the
weekend. For Late Thursday into Friday, an upper trough is forecast
to move through. Upper jet divergence is favored for our region. A
weak LLJ is shown to nose into the southern portion of Michigan,
especially by the 00z GFS and GEM. ECE and GEFS ensemble guidance
suggests PWATs rising into the 1.00"-1.25" range. With synoptic
scale lift, increasing moisture, and marginal elevated (and
potentially some surface based) instability present, showers and
thunderstorms remain in the forecast mainly from Thursday
evening/night into Friday. POPs are around 50% but likely trending
up as we get closer, once medium range guidance pins down the
timing.

Following this system, much depends on how strong the upper ridging
gets over the upper Midwest and Great Lakes, and whether a renewed
LLJ develops an orientation that brings in low level moisture and
forcing to the region. Various model solutions exist over the
weekend, leading to low confidence in the synoptic pattern
evolution. Ensemble guidance 50th percentile 24hr QPF currently
favors little in the way of rainfall, but several individual
ensemble members have some significant rain amounts over an inch.
With convectively driven precipitation, that is to be expected this
far out. NBM POPs of around 30% for the weekend look good. Highs in
the 70s are favored at this time.

&&

.AVIATION /12Z TAFS THROUGH 12Z TUESDAY/...
Issued at 742 AM EDT Mon May 13 2024

Shower and thunderstorm activity from earlier this morning left
the region, but additional development is likely later this
afternoon into evening especially. Timing individual storms even
at this point is difficult, and opted not to mention VCTS. The
greatest risk for TS will be after 21z. Model guidance has become
more pessimistic with regard to ceilings later tonight. Expect a
considerable lowering in the 00z to 06z time frame, and by 06z or
shortly thereafter, IFR is likely and LIFR is possible as well.
This threat is shown to also impact the 12z-15z Tuesday period,
just beyond this TAF update.

&&

.MARINE...
Issued at 244 AM EDT Mon May 13 2024

Lighter winds expected today and much of tonight before an
increase in winds occurs closer to Tuesday morning. Small Craft
Advisory still looking likely beginning early Tuesday.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...None.
MARINE...None.

&&

$$

UPDATE...NJJ
DISCUSSION...Maczko/Hoving
AVIATION...Hoving
MARINE...Maczko