Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 181148
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
748 AM EDT Tue Jul 18 2017

LATEST UPDATE...
Aviation

.SYNOPSIS...
Issued at 330 AM EDT Tue Jul 18 2017

A frontal boundary will meander through the region over the course
of the next several days bringing periods of showers and storms. The
first round may come late tonight into Wednesday morning, followed
by another late Wednesday night into Thursday. Rounds of showers and
storms will continue into the latter portion of the week and into
the weekend as well. It will certainly feel like summer right
through the weekend with highs in the 80s expected each day.

&&

.SHORT TERM...(Today through Thursday)
Issued at 330 AM EDT Tue Jul 18 2017

Main forecast focus was on trying to determine the more likely time
frames for showers/storms in the short term. An oscillating frontal
boundary will bring periodic chances for showers and storms although
there are a couple preferred times it appears. Summer like
temperatures are expected through Thursday with highs well into the
80s and lows in the 60s.

The first time that showers and storms are a better bet is late
tonight into Wednesday morning. All of the models have some sort of
low level jet into our area overnight. Instability is not all that
impressive, but the NAM is indicating a narrow area of +1000 j/kg
CAPE values sliding through the area. Expecting showers and storms
to fire in Central and Northern Wisconsin this evening. These
showers and storms will move into Lower Michigan in a fading fashion
overnight. In fact, the NAMnest showers the precipitation largely
falling apart. Scattered showers and storms in the forecast for now
with 30-50 pct pops. Precipitation will carry over into Wednesday
morning before tapering off. An isolated shower or storm is possible
Wednesday afternoon.

Wednesday night will likely start off dry with another round of
showers and storms moving in late. Models show another low level jet
nosing in late. so, expecting another round of showers and storms
late Wednesday night into Thursday. Better instability exists
Thursday afternoon, where CAPE values are forecast to reach
1000-2000 j/kg.

Bottom line, a round of showers/storms late tonight and Wednesday
morning followed by another late Wednesday night into Thursday. The
strongest storms in the short term will be possible Thursday
afternoon. Warm and increasingly humid.

.LONG TERM...(Thursday night through Monday)
Issued at 330 AM EDT Tue Jul 18 2017

Much of the period could be active with several frontal boundaries
moving through the cwa and several rounds of showers and storms.

The first round of showers/storms could come Thursday night but more
likely Friday. Both the ecmwf and gfs show an active warm/stationary
front Friday with a short wave interacting with the front over
southern Minnesota. This frontal boundary is progd to stretch from
southern Minnesota to northwest Ohio. This boundary location would
place much of the cwa in a position to receive pcpn. Roughly 40kts
of shear is progd through the period suggesting that some convective
organization is possible. Given the llj intersecting the frontal
boundary in Iowa, we may see several rounds of storms moving
southeast along the front through the cwa Friday into Saturday.
Precipitable water values remain right around 2 inches through the
period, so heavy rain may be a factor too. If we get training echos
we may be looking as some flooding issues Friday into Saturday and
maybe even Saturday night depending on the timing of this. The
models finally clear the pcpn out of the cwa by Sunday as the cold
front moves through. Sunday night and Monday look dry.

Highs will be in the lower to mid 80s Friday through Sunday before
falling back to the upper 70s Monday behind the cold front.

&&

.AVIATION...(For the 12Z TAFS through 12Z Wednesday morning)
Issued at 747 AM EDT Tue Jul 18 2017

VFR conditions are expected today under mostly sunny skies. Light
winds from the southwest are expected under 10 mph. Tonight,
clouds will be on the increase as showers and thunderstorms move
in off of Lake Michigan after midnight. Ceilings will likely be
around 4,000ft.

&&

.MARINE...
Issued at 330 AM EDT Tue Jul 18 2017

A light wind regime is expected today which with quiet conditions
out on the big lake. Winds increase tonight from the southwest in
response to a 30 knot low level jet around 2000ft off the ground.
Winds will be less at the surface, but there may be a brief kick up
in waves late tonight into Wednesday morning. Could see waves rise
into the 2 to 4 foot range tomorrow morning. Winds drop off again
for Wednesday and Wednesday evening. Another increase in winds are
possible into Thursday morning associated with some thunderstorms
that may be in the area.


&&

.HYDROLOGY...
Issued at 145 PM EDT Mon Jul 17 2017

Current river flow across the region is running anywhere from near
to much above normal. The Kalamazoo River basin is largely near
normal, with the Grand, Muskegon, and Saginaw River basins near to
above normal. For some locations, the upcoming pattern may lead to
above normal flows with perhaps within or above bankfull rises.

Several opportunities for thunderstorms exist beginning late
Tuesday night/early Wednesday morning right on through the
weekend due to a frontal boundary hovering around the region.
While many portions of this time frame will be dry, when it does
rain it could be quite heavy. Precipitable water values will perk
up to around 1.75" early Wednesday morning with possible storms
propagating from Wisconsin to Lower Michigan, but they could be
weakening in the process. Additional chances exist for the second
half of the week and weekend as precipitable water values
occasionally soar to 2.00" or higher, mainly Thursday and then
again this weekend. Where storms train and persist, localized
flooding is a possibility especially toward the end of the week.

&&

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

$$

SYNOPSIS...Duke
SHORT TERM...Duke
LONG TERM...04
AVIATION...Duke
HYDROLOGY...Hoving
MARINE...Duke



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