Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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334
FXUS63 KGRR 201836
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
236 PM EDT Sun May 20 2018

LATEST UPDATE...
Synopsis/Short Term/Long Term/Marine

.SYNOPSIS...
Issued at 236 PM EDT Sun May 20 2018

Cloudy and cool conditions will persist through Monday night.
After an area of rain dissipates this afternoon another round of
precipitation will spread northeast into much of Southwest Lower
Michigan Monday morning. Showers and thunderstorms will remain
possible through Monday night. Dry weather and warm temperatures
return Tuesday with highs Wednesday and Thursday approaching 80. A
chance of showers and thunderstorms returns at the end of the
week into Memorial Day Weekend.

&&

.SHORT TERM...(This evening through Tuesday night)
Issued at 236 PM EDT Sun May 20 2018

Boosted rain chances on Monday significantly based primarily on strong
consensus and consistency among the short range convection allowing
models. SPC Day 1 outlook through Monday morning does keep general
thunder for SW Lower MI associated with the onset of this activity
as it spreads northeast into the area.

We were earlier concerned about frost over our northeast zones
Monday morning, but now believe that cloudiness will mitigate that
threat.

The Day 2 outlook has general thunder for nearly all of Lower MI.
Would expect the best chances farther south but agree the potential
also exists farther north given that a 30+ knot southwesterly low level
jet should overspread most of our forecast area with robust theta-e
advection while an upper PV max brings height falls and cold advection
aloft. Destabilization should be sufficient for widespread showers
and some thunderstorms, but convection should remain elevated with
no severe weather expected.

Based on discussion with DTX, have also increased shower and
thunderstorm probabilities Monday night. Thanks for coordination
today DTX, IWX, and LOT.

.LONG TERM...(Wednesday through Sunday)
Issued at 236 PM EDT Sun May 20 2018

A high-over-low upper air pattern establishes over the western
United States by Tuesday. Predictability with the evolution of
these features is generally poor. It is therefore not surprising
to see a wide variety of model solutions towards the end of the
week that are predicated on how much the upper ridge breaks down
with the passage of a shortwave trough. Position and timing of any
resulting surface feature is quite uncertain, but it is noteworthy
that a sizable minority of ECE members bring convection into the
area Friday afternoon and some of those members develop substantial
antecedent convective instability upwards of 2000 J per kg
SBCAPE. However, certainty is too low to warrant anything more
than a chance of thunderstorms at this point.

&&

.AVIATION...(For the 18Z TAFS through 18Z Monday afternoon)
Issued at 141 PM EDT Sun May 20 2018

MVFR conditions will persist for parts of the region this
afternoon. Some drier air was dropping down from the north and
should support rising ceiling levels by late afternoon. An area of
rain moving in from the west is expected to weaken with time...but
it will likely cross several of the TAF sites through mid
afternoon.

A warm front will lift up from the south Monday morning. This will
bring increasing levels of moisture along with rain. Impacts are
expected to increase through the day. Some instability may move in
by the afternoon...but confidence on storms was not high enough to
put them in the forecast at this point.

&&

.MARINE...
Issued at 236 PM EDT Sun May 20 2018

Allowed SCA to end as scheduled at noon. We are looking at
increasing offshore winds Monday afternoon. Although wave growth
will be limited, there is a small chance that winds could briefly
become sustained above 20 knots near the lakeshore. This is too
uncertain to advise on at this point, but it will be something to
watch moving forward.

&&

.HYDROLOGY...
Issued at 1127 AM EDT Sun May 20 2018

Many rivers especially in southern Michigan are running much above
normal for this time of year, with lowland nuisance flooding still
occurring at several points. Most river levels are falling.
Showers on Monday have the potential to produce around a half inch
of rain on average in southern Michigan, though it wouldn`t be
surprising if some locations picked up closer to an inch.
Depending on the basins where any heavier rain may fall, this
could prolong the nuisance flooding. The middle part of the week
will be dry, then rain chances return toward the end of the week.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...None.
LM...None.
&&

$$

SYNOPSIS...TJT
SHORT TERM...TJT
LONG TERM...TJT
AVIATION...MJS
HYDROLOGY...CAS
MARINE...TJT



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