Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 200551
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
151 AM EDT Thu Jul 20 2017

LATEST UPDATE...
Aviation

.SYNOPSIS...
Issued at 330 PM EDT Wed Jul 19 2017

A large complex of thunderstorms will move in during the early to
mid morning hours Thursday and bring potential for heavy rain
frequent lightning and some damaging wind gusts. It will remain very
warm and humid Thursday through Sunday with potential for several
more rounds of thunderstorms mainly from Friday night through
Saturday night.

&&

.UPDATE...
Issued at 814 PM EDT Wed Jul 19 2017

The high resolution models continue to indicate a second area of
thunderstorms will develop over central Wisconsin behind the first
storm complex currently diving southeast across west central and
southwest Wisconsin. The second area of storms will reach our
forecast area after about 4 am and may have gusty winds and
locally heavy rain. As a result I updated the current zone package
to reflect that idea.

&&

.SHORT TERM...(This evening through Friday night)
Issued at 330 PM EDT Wed Jul 19 2017

Primary short term fcst concerns continue to involve determining
convective/svr wx/hvy rain potential through Friday night.

Active wx will develop early Thursday morning as an mcs rolls in
from the wnw. The mcs will develop over the upper midwest
(southern MN/WI) this evening on the nose of a strong llj. The
850-300 mb thickness pattern for MCS steering/propagation
suggests this mcs will move ese to se into our area during the
early morning hours Thursday.

In addition to frequent lightning and heavy rainfall there is
potential for the MCS to bring damaging wind gusts to portions of
our fcst area as strong 0-3km and deep layer shear is noted and the
MCS will probably evolve into one or several bowing segments. This
is the most likely scenario for our fcst area.

Though not the most likely scenario I am concerned about an outside
chance for a progressive derecho to develop over the upper midwest.
This could occur with mid/upper level winds parallel to the
stationary frontal boundary and sref guidance indicating the derecho
composite parameter will become very favorable over the upper
midwest late this evening. This notion is supported by progressive
derecho climatology and building heat over the central/eastern CONUS
as well.

A meso-high will begin to build in Thursday afternoon after the
MCS moves through which in conjunction with some lingering clouds
and a worked over atmosphere will result in minimal convective
potential for most of the aftn.

A consensus of short range guidance suggests that some instability
may develop by late aftn/early evening over our far southern fcst
area near the I-94 corridor where convection may redevelop. However
it seems the greater threat for convective redevelopment on old mcs
outflow boundaries will occur mainly just south of our fcst area.

Subsidence under a weak high pressure ridge should inhibit
convective development from Friday morning through most of the
afternoon. However a consensus of most of our latest short to
medium range guidance suggests convection will redevelop Friday
night to the north of a quasi stationary frontal boundary that
will be draped across northern IL/IN/OH by then.

This looks like a potentially very favorable setup for heavy rain
across portions of our fcst area as precipitable water values surge
to around 2-2.25 inches by 12Z Saturday. 1000-850 mb moisture
transport becomes increasingly favorable overnight. Some strong to
severe storms are also possible as elevated instability begins to
ramp up with favorable deep layer shear values of 35-45 kts.

As for late this afternoon... a few showers and perhaps a weak
isolated thunderstorm will continue to develop from near KAZO east
along the I-94 corridor.

.LONG TERM...(Saturday through Wednesday)
Issued at 330 PM EDT Wed Jul 19 2017

Wet and unsettled weather is expected to continue through Monday
with much drier conditions Tuesday and Wednesday. The main concerns
are thunderstorms with heavy rain on Saturday (possibly carried over
from Friday night) and the prospects for resultant flooding.

There are notable signals for heavy rain on Saturday. Several of the
last GEFS runs show QPF in the 95th percentile relative to its own
model climatology (also referred to as M-climate). This in theory
should account for the known wet and under-dispersive biases
associated with this particular ensemble. Even with this correction,
there remains a quite anomalous wet signal that has been apparent
for the last several runs. The NAEFS supports this possibility by
advertising very strong zonal flow in the lower troposphere and
seasonably high PWATs during this time.

Conditions will remain unsettled, but likely less organized, late
Saturday into Monday with a slowly approaching upper trough. As
noted before, seasonably cool and dry conditions are expected Monday
night into Wednesday as ridging at the surface and aloft build in
behind the exiting trough.

&&

.AVIATION...(For the 06Z TAFS through 06Z Thursday night)
Issued at 151 AM EDT Thu Jul 20 2017

A low confidence forecast exists for the first few hours dealing
with waves of showers and storms. One complex dissipated as it
approached Chicago a few hours ago, with another forming along the
Mississippi River near the WI/MN border. Latest radar trends
indicate that the new line is trying to propagate to the SE. These
lines also seem to be preventing more storms from forming further
to the N. It is still possible other showers and storms form and
impact the area from prior to daybreak through mid-day.

Any widespread showers/storms that might impact the area should
move out for the most part by afternoon. Can not rule out a pop up
shower/storm this afternoon with diurnal instability, but the
coverage will be fairly low.


&&

.MARINE...
Issued at 330 PM EDT Wed Jul 19 2017

Fairly minimal wind and wave action is anticipated late this
afternoon through Thursday. The exception to this is during the
early to mid morning hours Thursday when a strong low level jet
will result in a bump up to southwest winds of 15 to 25 mph and
waves to 2 to 4 feet.

In addition a large thunderstorm complex will bring potential for
strong gusty winds and briefly higher waves during the early to
mid morning hours Thursday.


&&

.HYDROLOGY...
Issued at 330 PM EDT Tue Jul 18 2017

Although locally heavy rain is likely with thunderstorms moving
through the region into Thursday morning, the progressive nature of
the storms will limit the flood potential. What these storms may do
is set the stage (via antecedent conditions) for a possibly bigger
hydrologic event Friday night into Saturday. It is possible that PW
values will be near the maximum July values that have been recorded
going back to the 1940s (reference: SPC sounding climatology page
for DTX). Those maximum values are in the 2.00"-2.25" range. GFS,
GEM, and ECMWF guidance are strongly indicating a large area of over
2.00" PW for southern and central Lower MI for approximately a 24
hour window from Friday night into late Saturday, with some
solutions indicating over 2.25". This would equate to high rainfall
rates within any convection and an increased flood potential, at
least on local scales.

In terms of river flood potential, the main stem rivers would need
quite a bit of rain to go above flood stage, based on some ensemble
QPF analysis for the next 72 hours. Smaller rivers and streams would
be more prone to reaching flood stage with this setup. Still,
depending on where the storms move, the larger basins could be at
risk for considerable rises and possible flooding at certain points.

&&

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

$$

UPDATE...WDM
SYNOPSIS...Laurens
SHORT TERM...Laurens
LONG TERM...TJT
AVIATION...NJJ
HYDROLOGY...Hoving
MARINE...Laurens


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