Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 191920
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
315 PM EDT Sat Aug 19 2017

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

.SYNOPSIS...
Issued at 315 PM EDT Sat Aug 19 2017

High pressure will continue to bring fair weather through the rest
of the weekend. A stationary frontal boundary will bring a few
showers and thunderstorms Monday with a better chance for showers
and thunderstorms Monday night into Tuesday with a cold front. It
will become warmer and more humid Sunday and Monday before
cooler weather returns for mid to late week.

&&

.SHORT TERM...(This evening through Monday night)
Issued at 315 PM EDT Sat Aug 19 2017

Primary short term fcst concerns involve determining probabilities
for showers and storms Monday through Monday night.

Fair wx will continue tonight through Sunday night with high
pressure continuing in control of the wx pattern. It will become
warmer and little more humid Sunday with ample sun and southerly
winds helping to boost high temps into the middle to perhaps upper
80`s.

Dry wx will continue for most of the day Monday but increasing
moisture and a stationary frontal boundary in our area will focus
development of a few showers and isolated thunderstorms.
Conditions for viewing the eclipse in our fcst area appear
unfavorable as a consensus of 12Z time height rh progs and
numerical guidance all suggest that there will likely be fairly
extensive cloud cover around.

A better chance for showers and thunderstorms will come Monday night
with the approach of the sfc low and cold front and additional
forcing from a 30-35 kt llj. No severe wx is anticipated mainly due
to decreasing instability overnight but some locally heavy rainfall
is possible given increasing moisture/high precipitable water values
ahead of the cold front.

.LONG TERM...(Tuesday through Saturday)
Issued at 315 PM EDT Sat Aug 19 2017

There are two issues with this forecast, first of course is the
convective rain event Tuesday, then how much cooler it gets beyond
that (not unusual for this time in August).

We have been watching for the past several days the potential for a
significant convective and rainfall event on Tuesday. SPC has us in
the 5 day outlook yesterday and the day 4 outlook today. Both WPC
and our grids suggest between .75 inches and around 1.5" of rain
from this event (mostly late Monday night into Tuesday afternoon). A
quick check of model continuity for both the ECMWF and GFS tell me
the models have been forecasting this Tuesday event since the 17th
at 12z.  Not the best possible continuity but with every run of both
the ECMWF and GFS since the 17th at 12z suggesting a very similar
event that is a good and believable trend.

Dynamically we are in the right entrance region of the digging upper
jet. That is something we have not seen a lot of this year actually
but it is a great place of you want strong thunderstorms and heavy
rain. Adding the Precipitable water values on both models around 2".
That 2 inches is close the max line the SPC sounding page for this
time of year. Going along with this the CIPS analogs show  the best
area for one inch or more of rain for Tuesday to be near and south
of I-96. CIPS also shows the severe storm event frequency to be near
60% east of BTL and south of I-96. This area for severe storms is
also backed up by the SPC SREF 3 hour calibrated probability of
severe storms. All of this fits well with the concept of digging
upper jet core putting Southwest Michigan in the jet entrance
region.

Given the continuity and favorable position of the upper level jet
and low level jet for this event in combination of near record
amounts of precipitable water. it should be of little surprise we
have conditional pops for a significant part of the CWA Tuesday.

Beyond that the northern stream continues to dig and put most of
eastern Canada and Ne quarter of the CONUS in cool air most of next
week. This partly due to the reloading of the system in the Gulf of
Alaska, allowing the western ridge to persist, which in turn leads
to troughing over the eastern CONUS.  Since we are now more than
midway through August I want to point the calendar frequency for
high temperatures of 80 degrees or more falls below 50% on the 24th.
So having strings of days with highs below 80 will continue to
become much more frequent. In GRR in August of 2015, after the 20th
of August we had only 2 days with highs of 80 or more (actually 81
degrees on the 22 and and 31st).  Even last year, 2016 we had only 1
day after the 20th with highs of 85 or more in August (85 on the
28th). So this turn toward cooler is typical of this time of year.

The bottom line is thunderstorms with locally heavy rainfall
Tuesday, then clearing and much cooler Wednesday into Saturday.


&&

.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
Issued at 1159 AM EDT Sat Aug 19 2017

VFR conditions are expected into this evening then some MVFR
fog/mist early Sunday morning only to become VFR once again after
sunrise.

There is an upper trough/shortwave moving east of the
area as I write this. There is unstable air assoicated with that
trough but since it is already east of here, I would think any
convection from it would be east of I-75 (for the most part). This
can be seen on the the VIS satellite loops over MI with the
enhanced CU in the Thumb are to near western Lake Erie. The area
from the Thumb to near Jackson will have around 500 j/kg of most
unstable cap this afternoon. So, east of Jackson a storm is
possible.

Tonight high pressure moves overhead and winds should be near calm
at the surface with clear skies so light fog is expected. Tomorrow
the upper ridge moves through the area and surface winds turn to
the south so it should be solid VFR once the morning fog mixes
out.


&&

.MARINE...
Issued at 315 PM EDT Sat Aug 19 2017

Wind speeds/wave heights through the weekend will be minimal.
Tranquil conditions will continue Monday but thunderstorms will
pose a hazard to mariners late Monday through midday Tuesday.
Winds and wave heights will ramp up late Tuesday and Wednesday as
brisk northwest winds advect a much cooler airmass into the
area.

&&

.HYDROLOGY...
Issued at 315 PM EDT Sat Aug 19 2017

Our hydrologic focus remains on early this coming week. Confidence
is increasing that Monday into Tuesday could feature locally heavy
rains with a favorable setup for thunderstorm activity across our
region, more so than many of our events that we have had this
summer. Medium range guidance is generally indicating a potential
for 1" or more of rain with this system, with pockets of less than
1" mainly east of US 131. Keep in mind that medium range models are
non-convection allowing so these amounts will likely be locally
underdone. With PWAT values once again approaching 2.00" and
favorable H300 jet divergence / H850 LLJ of 30-40 kts and a sfc
frontal boundary draped across the Lower Peninsula, it is not hard
to envision localized flooding (ie. some water covered roads) with
this system. River responses depend on placement of rainfall and
duration of heavy rains. Small rivers and streams will be most
susceptible to quick rises.

&&

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

$$

SYNOPSIS...Laurens
SHORT TERM...Laurens
LONG TERM...WDM
AVIATION...WDM
HYDROLOGY...Hoving
MARINE...Laurens


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