Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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873
FXUS63 KGRR 301143
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
743 AM EDT Sun Apr 30 2017

LATEST UPDATE...
Aviation

.SYNOPSIS...
Issued at 326 AM EDT Sun Apr 30 2017

Expect periods of rain with embedded thunderstorms today and
tonight. Winds will be strong and gusty. This is all thanks to a
strong and deepening occluded low pressure system that will track
from eastern Kansas this morning to central Wisconsin Monday.
Temperatures will stay chilly near and north of Grand Rapids
through tonight but I-94 the warm front may bring temperatures
into the mid to upper 60s by this evening. Rainfall amounts of
over 2 inches are expected by Monday morning. There could be some
strong to possibly severe storms and an isolated tornado south of
I-96 this evening as the warm front comes into our southern
sections before the occluded front moves through.

From Monday afternoon through Tuesday expect occasional rain
showers windy and chilly temperatures. highs will struggle to make
50 degrees on Tuesday.

&&

.SHORT TERM...(Today through Tuesday)
Issued at 326 AM EDT Sun Apr 30 2017

I will continue the flood advisory as is even through the
rainfall last night was not quiet as high as expected, even so we
had between a half and three quarters of an inch by 2 am over most
of Southwest CWA. We expect periods of showers and thunderstorms
into tonight with additional rainfall of 1 to 3 inches.

The other issue is the potential for strong thunderstorms with
isolated tornadoes this evening near and south of Interstate 94 as
the occluded front moves through that area. There is decent cape,
a low LCL, more than 40 knots of effect bulk shear in that area
this evening. This will have to be watched closely as we
typically get tornadoes near warm fronts, this is that sort of set
up.

Beyond this we get into the cold air Monday afternoon and the DGZ
is near the top of the clouds with strong cyclonic flow, Monday
afternoon through Tuesday afternoon. This means lake enhanced rain
showers from late Monday afternoon through Tuesday. It will be
breezy and cold then too.

.LONG TERM...(Tuesday night through Saturday)
Issued at 326 AM EDT Sun Apr 30 2017

We`re mainly looking at dry weather during the long term. High
pressure over Lower Michigan will likely lead to some frost Tuesday
night, Wednesday night, and Thursday night. Frost/Freeze headlines
may be needed, especially for the central and northern cwa. We added
frost to the grids each of those three nights.

A deep upper trough is progd to develop east of the Plains during
the period. The sfc low is a southern stream system that will bring
rain to the Ohio Valley mid to late week. The ecmwf tracks the low
northeast through Pennsylvania during which time pcpn retrogrades
westward a bit through southeast Lower. Under this scenario, it`s
possible that the far southeast cwa may see a shower Friday and
Friday night, but probably not.

Temperatures will be cool through the period, but moderate somewhat
toward the end of the week with highs mainly in the lower 60s.

&&

.AVIATION...(For the 12Z TAFS through 12Z Monday morning)
Issued at 744 AM EDT Sun Apr 30 2017

Expect largely IFR conditons until the cold front comes through
after 09z. Until then there will be periods of showers and
thunderstorms. During the evening as the upper level jet comes
over Southwest Michigan. Which implies between 00z and 06z is
when thunderstorms would be most likely since that is when the
forcing is the greatest. Once the main shortwave comes through I
would expect mostly rain showers till the front actually comes
through.

During the day today there will be a very strong inversion between
12z and 18z around 5000 ft agl. That will help to keep the
ceiling in the IFR range. During the afternoon the inversion will
get closer to the ground, that is, as the warm front at the
surface, gets closer to our SW MI taf sites. Below the invasion
winds will be from 120 degrees around 35 knots above the inversion
winds will be 210 degrees around 35 knots. Clearly with 2000 ft
of the front (vertically) there will be a significant wind shear.


There may even be a brief break in the clouds (sky goes clear)
behind the cold front but I did not try to put that sort of detail
in the forecast as this point.


&&

.MARINE...
Issued at 326 AM EDT Sun Apr 30 2017

I have replaced the gale warning with a small craft advisory
through early Tuesday morning. Likely it will be extended to
Tuesday night.


&&

.HYDROLOGY...
Issued at 122 PM EDT Sat Apr 29 2017

Flood advisories continue for the Grand River at Ionia and on the
Maple River at Maple Rapids. A flood watch continues for the same
four points. River statements have been issued for numerous
locations that might experience flooding towards the middle of the
coming week. However, forecast certainty for these sites remains
very low at due to high dependence on the character of precipitation
that occurs over the next 48 hours and how well the models handle
the subsequent river crests.

It is quite certain, however, that widespread rainfall affecting all
area river basins will fall between now and Monday morning.  The
latest round of river forecasts were based on 48 hour WPC QPF, which
is slightly greater and shifted farther north than WFO QPF. SREF
guidance shows mean values around 2", although there are a couple
outliers with 4+", making the median QPF a little bit below 2".
Interestingly, downscaled calibrated GEFS output shows only a 40-50%
chance of exceeding 2 inches in most spots. However, we favor the
ECE, whose ensemble members show strong consensus squarely in the 2-
3" total QPF range, in line with previous thinking. As noted
already, local maxima in precipitation are likely but there is
little predictability with exactly where they will occur. This of
course has potentially significant implications with what river
basins are most affected.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...Flood Watch through Monday morning for MIZ043>046-050>052-
     056>059-064>066-071-072.

LM...Small Craft Advisory until 5 AM EDT Tuesday for LMZ844>849.

&&

$$

SYNOPSIS...WDM
SHORT TERM...WDM
LONG TERM...04
AVIATION...WDM
HYDROLOGY...TJT
MARINE...WDM



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