Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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467
FXUS63 KGRR 230852
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
452 AM EDT Mon Oct 23 2017

LATEST UPDATE...
Mesoscale Discussion/Marine
Corrected valid times for short and long term sections

.MESOSCALE DISCUSSION...
Issued at 452 AM EDT Mon Oct 23 2017

GRR and APX radars already indicate a pronounced southwest to
northeast fgen band of enhanced precipitation that extended from
between Ludington and Big Rapids to Gaylord. A home weather
station under the primary band west of Cadillac recently reported
over an inch of rain in an hour, so we are already seeing
efficient rainfall processes at work. The blended satellite
derived total precipitable water product shows a well defined
plume of moisture extending all the way from the Gulf of Mexico
into western MI with estimates of 1.4" in the region of heaviest
rain. This is corroborated by the 1.42" measured with the APX raob
Sunday evening, which is in record territory for this time of
year.

SPC mesoanalysis graphics suggest the strongest frontogenesis is
roughly in the 850-700 mb layer which allows our radars to detect
the heavier precipitation at greater range than normal. The HRRRX
suggest that this fgen area and associated precipitation will
move little through the rest of this morning, so we could already
see significant rainfall accumulations over the northwest
forecast area before noon.

&&

.SYNOPSIS...
Issued at 317 AM EDT Mon Oct 23 2017

Rain, occasionally heavy, will spread across the area this afternoon
and last much of tonight. Rain will continue Tuesday before
tapering off Tuesday night. Rain showers will be less frequent for
the latter half of this week. Temperatures will be seasonably
cool with highs in the upper 40s to low 50s for much of the week.
It will feel blustery Tuesday and Wednesday with gusty winds.

&&

.SHORT TERM...(Today through Tuesday)
Issued at 452 AM EDT Mon Oct 23 2017

Have expanded the flood watch westward to encompass the entire forecast
area. The focus for the heaviest rain has shifted northwest and
there now may be a reduced threat for heavy rain along the US-127
corridor around Lansing and Jackson. We will keep this area in the
watch however given uncertainty in the details. It is looking
increasingly likely that a sizable part of the forecast area could
receive 4-5 inches of rain, most of which will fall from this
afternoon into Tuesday morning, which could impact commutes this
evening and especially Tuesday morning. There are indications
that significant rainfall accumulations could continue well into
the day Tuesday, boosting totals even further.

We are close enough in time to the event to utilize convection allowing
models. Not surprisingly, they advertise much greater QPF due to better
depiction of the impressive fgen that is too narrow to be properly
resolved by the global models. It`s looking more likely that the
deformation/fgen area will rotate over the forecast area this
evening. This will result in prolonged durations and greater coverage
for rainfall. With the rotation of the mid-level thermal gradient comes
a significant Fs contribution to the mid-level frontogenesis further
boosting upward vertical motion and precipitation rates. Atmospheric
moisture remains rather extreme for this time of year. The 00Z
APX and GRB soundings were 265 percent and 185 percent of normal,
respectively. Therefore the environment supports intense rainfall
rates.

A peek at the 23/00Z ECMWF reveals that additional rainfall Tuesday
could be significant particularly to the northeast in the vicinity
of Mount Pleasant and Clare. In fact, these areas may end up with
the greatest totals by the middle of the week.

.LONG TERM...(Tuesday night through Sunday)
Issued at 452 AM EDT Mon Oct 23 2017

Rain showers will be diminishing in coverage Tue night, however they
will not quite come to an end until probably Wed night. The sfc low
and associated wrap around pcpn will shift out, taking the heavier
rainfall with it. The long wave upper trough will hold in across the
area, and another short wave will dive SE over the area on Wed.

We will likely see a reprieve in the rain showers on Thu as we will
be sitting in between storm systems. A short wave upper ridge will
control the weather between the departing low, and the next low
poised to move in from Canada. Temperatures will try to recover some
with a solid srly gradient ahead of the next system, and some
possible sunshine.

Rain shower chances will reappear on Fri, and will linger off and on
through the weekend. The next cold front will pass through early
Friday, with the secondary cold front with the upper wave coming
through Friday evening. A break looks possible late Sat and early
Sun before yet another upper low digs SE across the area. These
showers have the potential to mix in with some snow at night with
temps dropping into the 30s and freezing heights approaching the
sfc.

&&

.AVIATION...(For the 06Z TAFS through 06Z Monday night)
Issued at 144 AM EDT Mon Oct 23 2017

A wet period with significant aviation impacts due to IFR/LIFR
conditions can be expected over much of the next 24 hours. IFR has
already arrived at KMKG and KGRR with rain showers with the
passage of the front. This will continue for the next couple of
hours, and will then start to creep to the SE. It will take a
while before IFR finally gets to KJXN, like toward 19z or so.
Cigs will remain rather stable once the IFR settles. Vsbys will
vary a bit depending on the intensity of the showers that move
through. The bottom line though is that conditions will be
IFR/LIFR.

We will see conditions improve slightly with regard to cigs late
in the period as the low moves far enough north to take the lowest
conditions with it. We are looking MVFR moving in toward the end
of the period. Winds will also come up as the low starts to move
away. We will start to see gusts in the 20-25 knot range also
later in the period.

&&

.MARINE...
Issued at 452 AM EDT Mon Oct 23 2017

Based on latest WW3 guidance and in coordination with other
offices, went with a small craft advisory areawide beginning
this evening and a gale watch areawide beginning Tuesday morning.

&&

.HYDROLOGY...
Issued at 310 PM EDT Sun Oct 22 2017

Showers will spread into the area from the west through this
evening, with coverage of rain filling in overnight. This looks to
bring about a half-inch of rain to west Michigan by about 8AM Monday
morning. Rain may be moderate in intensity at times, but is not
expected to cause flooding overnight.

There should be a brief decrease in shower activity Monday morning
before another round of rain develops over Lower Michigan Monday
afternoon and lasts into Tuesday. Most locations look to receive at
least an additional inch of rainfall, but a north-to-south
orientated swath of heavier rain, with amounts of possibly 3 to 5
inches, is expected over the Lower Peninsula. At this time, it looks
like this heavier swath of rain will be located somewhere between
U.S. 131 and the Detroit-area. A Flood Watch therefore remains in
effect for counties generally east of U.S. 131. Localized flooding,
ponding on roadways, and rises in rivers, creeks, and streams could
occur within the area of heaviest rain. Confidence in the location
of the heaviest rain is not high, and refinements to the forecast
are still possible.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...Flood Watch from 2 PM EDT this afternoon through Tuesday morning
     for MIZ037>040-043>046-050>052-056>059-064>067-071>074.

LM...Gale Watch from Tuesday morning through Wednesday morning for
     LMZ844>849.

     Small Craft Advisory from 6 PM this evening to 10 AM EDT Tuesday
     for LMZ844>849.

&&

$$

MESOSCALE...TJT
SYNOPSIS...TJT
SHORT TERM...TJT
LONG TERM...NJJ
AVIATION...NJJ
HYDROLOGY...HLO
MARINE...TJT



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