Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 211913
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
310 PM EDT Wed Jun 21 2017

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

.SYNOPSIS...
Issued at 310 PM EDT Wed Jun 21 2017

A warm front will cause showers and thunderstorms to develop late
tonight. It will become very warm and humid Thursday and
thunderstorms will redevelop late Thursday through Thursday night
along and out ahead of an approaching cold front. It will turn
much cooler this weekend into early next week.

&&

.SHORT TERM...(This evening through Friday night)
Issued at 310 PM EDT Wed Jun 21 2017

Primary short term fcst concerns continue to involve assessment of
convective/severe wx potential late tonight through early Friday.

The combination of a northward moving warm front and strong llj
will cause showers and thunderstorms to develop after midnight
tonight. Severe wx overnight is unlikely but isolated strong to
marginally severe elevated storms may contain hail in addition to
locally heavy rainfall.

After warm frontal passage and early morning convection moves out
it will become very warm and humid Thursday with high temps
reaching the mid to upper 80`s with dew points rising through the
60`s. We expect most of the period from mid to late Thursday
morning through most of the afternoon to be dry.

An approaching cold front will result in development of strong to
severe storms late Thursday afternoon through Thursday night.
There are many favorable ingredients for strong to severe
convection including strong deep layer shear on the order of 40-55
kts in conjunction with ample low level moisture and forcing from
the front.

One of the primary limiting factors for more robust convective
development may turn out to be instability which is progged to be
weak to moderate by most shorter range computer model guidance.
Cold frontal timing is also not the most conducive for severe
storms. Finally upper level dynamics/forcing are not impressive.
Strong pva does not come in until Friday after fropa has already
occurred. All things considered though it does seem that some
strong to potentially severe storms will develop late
Thursday/Thursday evening through Thursday night.

Locally heavy rainfall is possible late tonight (06Z-12Z) mainly
across our northern fcst area as there is moderate moisture
transport and pwat values will reach around 1.75 inches. More
widespread heavy rainfall is anticipated Thursday night with strong
1000-850 mb moisture transport and pwat values near two inches by
then.

Skies will gradually clear Friday after frontal passage and a much
cooler and drier airmass will gradually advect in Friday night.

.LONG TERM...(Saturday through Wednesday)
Issued at 310 PM EDT Wed Jun 21 2017

Confidence is high that the long term period will feature below
normal temperatures due to persistent troughing aloft. That
typically means a comfortable temp/humidity pattern for this time of
year in southwest MI.

This pattern supports an almost daily risk of a shower due to the
possibility of being impacted by shortwaves embedded in the
northwest flow. Also the cool air aloft aids diurnal cu development.
However the timing, track and frequency of shortwaves/vort maxima is
always difficult in this type of pattern, which makes daily pops a
challenge, so for now generally only low chc pops in any period.
Most locations probably won`t see much QPF in the long term period.

The GFS is quicker than the ECMWF in lifting out the upper trough,
and tend to believe the ECMWF, but either way each solution suggest
at least the beginning of a moderating trend in temps by Wednesday.

&&

.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 200 PM EDT Wed Jun 21 2017

A period of thunderstorms is expected late tonight with an
incoming warm front, starting around 06Z and continuing through
12Z. Confidence is high enough to go with about a 3-4 hour period
of predominate thunder at MKG/GRR/LAN, but confidence is lower
farther south so will carry only VCTS at AZO/BTL/JXN overnight.

Once the warm front goes through early Thursday morning, the
chance of storms will decrease and winds will increase out of the
southwest to 15 to 25 knots and become gusty. Outside of the
period of storms tonight when heavy rain could bring brief IFR or
lower, VFR weather is expected.

The next chance of storms after tonight should hold off until
after 18z Thursday, and probably even more toward 00z Friday.
These storms could be strong/severe with aviation impacts.


&&

.MARINE...
Issued at 310 PM EDT Wed Jun 21 2017

Wave heights and wind speeds will remain minimal through this
evening. We have hoisted a small craft advisory for all our
nearshore zones from early Thursday morning through Thursday
afternoon.

Wind speeds by early to mid morning should reach 20 to 25 kts and
waves may build a little higher than wave model guidance suggests
given the strength of the llj. Thunderstorms will pose a hazard
to mariners late tonight and again for late Thursday through
Thursday night.

&&

.HYDROLOGY...
Issued at 310 PM EDT Tue Jun 20 2017

The uncertain extent of coverage, location, and repetition of
thunderstorms Thursday morning and Thursday night makes the
rainfall forecast difficult. However, with PW values between 1.5
and 1.75 inches favoring efficient rain production (above the 90th
percentile for this time of year and approaching some June daily
record values in the DTX-area sounding climatology), and a tendency
for storms to repeat over parts of the area, it seems reasonable
that some spots could receive over 2 inches of rainfall. The most
favored area for significant rain totals will be north of I-96 and
perhaps closer to US-10.

The tendency for flooding to develop will depend on local antecedent
conditions. While May and early June were rather dry, last week`s
rainfall ranged from 1.5 to locally over 5 inches in areas north of
South Haven, Hastings, and Lansing. Locations that saw heavy rain
last week will be more prone to have at least minor areal or street
flooding from heavy downpours on Thursday. As for rivers, the Rogue
near Rockford, the Flat near Smyrna, and the Maple near Maple Rapids
are running high after last week`s rain, and will be less tolerant
of any additional heavy rain that falls within their basins. Above
bankfull rises are possible, so people with interests along those
rivers should monitor the situation.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...None.
LM...Small Craft Advisory from 6 AM to 6 PM EDT Thursday for
     LMZ844>849.

&&

$$

SYNOPSIS...Laurens
SHORT TERM...Laurens
LONG TERM...Meade
AVIATION...Meade
HYDROLOGY...CAS
MARINE...Laurens



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