Area Forecast Discussion
Issued by NWS Gaylord, MI

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FXUS63 KAPX 200404

Area Forecast Discussion
National Weather Service Gaylord MI
1204 AM EDT Thu Jul 20 2017

Issued at 1020 PM EDT Wed Jul 19 2017

Still quite a bit of uncertainty in the convective evolution
tonight, but maybe a few more signs are leading to a shade more
confidence in trends. Certainly the MCV and convective complex
racing through nrn IL is taking part in robbing nrn Michigan of
any early evening moisture transport from developing LLJ from Iowa
into the upper Mississippi valley. This storm complex will be
dropping SE with time, opening up a window for the WSW H8 LLJ to
at least try and advect moisture toward us, up and over a nearly
stationary frontal zone from SD to nrn IL. Renewed storms were
already firing north of this front, in srn MN. Problems for us
seeing precipitation revolve around the fact that the front is
expected to be nearly stationary through the night, while the H*
LLJ veers more westerly with time. This is likely to feed most of
the moisture and instability into areas south/SW of nrn Michigan.
Latest hi res data trying to suggest the same thing. Now we are
suggested to still have some of the synoptic/dynamic support aloft
with the arrival of a shortwave and upper divergence, but am
thinking that we may just see some showers roll through with
slight chance for thunderstorms. Convection is tough. Will see how
it plays out.


.NEAR TERM...(Tonight through Thursday)
Issued at 312 PM EDT Wed Jul 19 2017

...Rain chances on the increase later tonight...

High impact weather potential...Will need to monitor late night and
early morning thunderstorm potential/intensity.

Pattern synopsis/forecast...High pressure centered directly overhead
this afternoon, bringing sun-filled skies and warm temperatures
along with it. However, fast and energetic northern Conus flow
regime will bring quick changes late tonight, with intense moisture
advection already well underway to our west in region of strong
upper jet dynamics and mid level shortwave trough support. Sharp
instability gradient only further intensifying with time this
afternoon over the Northern Mississippi Valley, and looks to be the
focus for rapid development of a mesoscale convective system (MCS)
this evening. While this is a pretty high confidence scenario, what
follows is anything but with conflicting signals between evolution
of best upper/mid level support and placement of most intense
instability gradient resulting in plenty of uncertainty on
evolution/propagation of this MCS late tonight into early Thursday

Primary forecast concerns/challenges...Focus almost entirely on
shower/storm evolution later tonight into early Thursday morning.

Details...Challenging forecast for sure, all tied to uncertainty of
eventual movement of that anticipated MCS. Strongest synoptic scale
support centers itself across northern lower Michigan early Thursday
as robust right entrance region upper jet dynamics move overhead.
Mid level support will become increasingly convectively agitated
with time, but primary shortwave trough also is set to race across
northern lower toward morning. Alignment of greatest instability
gradient and placement of weak surface front do not do the same,
both of which stay focused to our southwest and south. Still believe
most organized and intense part of the convective complex will
follow the latter, with more broad and lighter showers/embedded
storms within waa wing of MCS and under core of stronger synoptic
support moving across northern lower very late tonight and early
Thursday morning (greater coverage the further south one goes in our
area). Given trends, expecting just a few light showers making it
into eastern upper. Now, if by chance a further north path to
primary storm complex is realized, effective shear exceeding 40
knots and what may be a well organized convective line along intense
cold pool could definitely bring a severe wind threat along with it.
Latest SPC Day 2 convective outlook has all of northern Michigan
placed under Marginal severe weather wording to account for this
possibility. Something to monitor for sure.

What is left of MCS expected to exit stage right by later Thursday
morning, with subsidence and drying taking place in its wake. Maybe
just enough forcing/moisture to allow for the redevelopment of a few
afternoon showers down near Saginaw Bay, but overall trends are
definitely for a drier afternoon than initially anticipated.
Temperatures just a bit cooler than those we are currently
experiencing, but still warm with afternoon readings near normal


.SHORT TERM...(Thursday night through Saturday)
Issued at 312 PM EDT Wed Jul 19 2017

...More shower and thunderstorm chances...

Nearly zonal flow aloft continues through the period. This will open
up the area to possible mesoscale convective complexes. Temperatures
will remain warmer than normal for later July. The main forecast
concerns revolve around pops Friday night into Saturday.

Thursday night into Friday...Surface high pressure is expected to
result in precipitation free conditions. Fairly comfortable at night
with lows ranging from the lower 50s to around 60. Highs in the
seasonably warm upper 70s to middle 80s.

Friday night into Saturday...Things may get a little dicey because
of a possible wave moving through the flow which a majority of
models have depicted. Looks like a mesoscale convective complex will
form once again but models vary on the track, some are to our south
while others bring it across our forecast area. Still too far out to
determine details so chance pops will be the safest course of action
at this point. Winds aloft are decent (0-6km bulk shear 40 to 50
knots) so there is potential for a couple severe thunderstorms
(mainly across southwest zones where better instability is shown to
be). Lows in the upper 50s to lower 60s. Highs Saturday are a little
tricky because of cloud/precipitation questions. It`s still pretty
warm aloft so a decent amount of sun could send temperatures upward
fairly quickly. Will be a little conservative with highs in the
upper 70s to lower 80s.

.LONG TERM...(Saturday night through Wednesday)
Issued at 312 PM EDT Wed Jul 19 2017

he 500mb pattern shows a trough dipping down from Canada over the
great lakes region Sunday into Monday...before flow returns zonal
again through the midweek period.  Closer to the area
of low pressure swings through Saturday night into Sunday...with
some wrap around affects straggling into Monday, before transient
high pressure regains control and settles things back down for the
remainder of the forecast period.  The timing of POP`s will continue
to be the forecast challenge for this weekend...and the moisture
supply is not overly impressive with any of the aforementioned.
However, if I had to pin down the most likely shot at any
shower/thunderstorm activity, it would be Saturday night through
Sunday.  The thing to watch will be the wrap around clouds/moisture
into Monday and whether that will materialize (as the GFS suggests)
or strip out more expediently (as the euro supports). Either way,
temperatures will drop to below normal for the new work week...with
highs generally in the 70s.


.AVIATION...(For the 06Z TAFS through 06Z Thursday night)
Issued at 1200 AM EDT Thu Jul 20 2017

Quiet conditions continue into the overnight hours as high
pressure crosses the region. Much uncertainty still on what
transpires very late tonight into Thursday, with at least some
support for an area of showers and maybe some rumbles of thunder
to move across the airports. Trends appear to lead to lesser
chances for rainfall, but given the uncertainty, will still carry
VCSH in the 09Z to 15Z timeframe. May be looking at a relatively
brief period of MVFR CIGS in the morning. Mostly light winds
through this forecast, with a sfc trough crossing after the
possible rains, for a shift out of the W/WNW.

There is at least some potential for low end LLWS tonight as the
possible showers arrive. Not a strong enough signal to mention in
the forecast right now.


Issued at 312 PM EDT Wed Jul 19 2017

Light winds tonight increase just a bit out of the
southwest on Thursday, although still look light enough to allow
some local afternoon lake breeze development. Will definitely need
to keep an eye on thunderstorm evolution late tonight and early
Thursday, with any storms that do develop capable of producing very
gusty winds and enhanced wave action.

Light winds expected to last through the remainder of the week into
at least the start of the weekend.




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