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Area forecast discussion 
National Weather Service Grand Rapids Michigan
909 PM EDT Friday Jul 25 2014

Latest update...

issued at 300 PM EDT Friday Jul 25 2014

The arrival of a warmer and more humid air mass will bring a threat
of a few showers and thunderstorms this weekend. Saturday looks like
the drier of the two days... with the better chance of rain on
Sunday afternoon and evening. The primary risk of severe
thunderstorms over the weekend will be south of Michigan.

Much colder weather will move in for Monday with highs struggling to
reach 70. Temperatures most of next week will remain below normal
with a risk of a shower or storm mainly inland from Lake Michigan.


issued at 907 PM EDT Friday Jul 25 2014

Updated to remove probability of precipitation this evening and lower them overnight. An
area of negative vorticity advection will move through the County Warning Area over the next few hours
prior to a weak short wave later tonight. Very little in the way
of upstream precipitation on radar at this time.


Short term...(this evening through Sunday night)
issued at 300 PM EDT Friday Jul 25 2014

The main forecast challenge is convective potential and timing over the
weekend. The best risk is on Sunday afternoon and evening with the
arrival of a strong shortwave dropping down from the northwest.

12z models have backed off a bit on probability of precipitation/quantitative precipitation forecast tonight... likely
related to the lack of a focused low level jet and relatively weak
moisture transport. Will keep probability of precipitation in the chance category.

Instability increases on Saturday with the arrival of surface dew points
in the middle to upper 60s. Sb convective available potential energy over 2000 j/kg are prognosticated in the
afternoon inland from Lake Michigan... which combined with deep
layer shear over 40 kts could support some healthy storms.

However an impetus for convective initiation over SW Michigan on Saturday
is very questionable since the surface front should be just south of the
Michigan/in border. Only chance/slight chance probability of precipitation seem warranted with the best
surface convergence expected to be located to our south and also to our
east across eastern lower Michigan.

Convective potential on Saturday night is not good either due to the
lack of a low level jet/forcing... despite good instability with mu
convective available potential energy over 2500 j/kg.

Convective potential does however increase on Sunday as approaching strong
shortwave induces a surface low/trough over lower Michigan. Combo of 500 mb height
falls... strong surface convergence... and convective available potential energy of 1500-2000 j/kg will
make thunderstorms likely. A severe weather threat could develop south and east of
GRR where best deep shear over 35 kts is prognosticated. Deep layer shear
on Sunday decreases farther north and west.

Long term...(monday through friday)
issued at 300 PM EDT Friday Jul 25 2014

Temperatures will average at least 5 to 10 degrees below normal
early next week as the upper level trough continues to amplify over
our region. Some mainly diurnally driven showers and perhaps a few
storms are possible due to instability from daytime heating and with
the pool of cold air aloft under the upper level trough.

Temperatures may moderate just a bit toward middle to late next week but will
continue to average a little below normal as the upper level trough
remains firmly entrenched over the Great Lakes region. There is a
continued chance of a few showers mainly near to east of US-131
mainly during the afternoon/early evening hours daily.


Aviation...(for the 00z tafs through 00z Saturday evening)
issued at 655 PM EDT Friday Jul 25 2014

Showers will track through the region into the overnight...but the
potential for MVFR or lower conditions looks lower. VFR conditions
prevailed so far. Trends in the precipitation support weakening.
Thus I will back off on the MVFR potential tonight. Still needs to
be monitored. The location that is most at risk for the low cloud
bases is kmkg. A small potential for IFR there after 07z. As for
Sat...while moisture is appears that clouds bases
will be somewhat more elevated. For now I have increased them to
2500 feet...but that may be too low.


issued at 300 PM EDT Friday Jul 25 2014

Guidance has not been consistent or in good agreement regarding the
potential for strong south winds over Lake Michigan tonight and Saturday
morning. The latest 12z guidance is not as aggressive as the 00z
guidance... so will tone down winds and waves a bit for tonight.

There is very high confidence in a strong north flow event over Lake
Michigan on Sunday night and Monday which will require small craft
advisories and beach hazard statements.


issued at 1206 am EDT Friday Jul 25 2014

After a stretch of mainly dry conditions for the majority of the area
we will see multiple chances of rain over the next several days.
One round will develop and move through late tonight and into
Saturday morning while the another round will develop Sunday
afternoon and evening. Area wide...totals should average out
around one-quarter to one-half inch through Sunday. Localized heavier
rainfall is expected in and around thunderstorms where amounts are
likely be be higher.

Additional chances for rain exist going into next week. At this
point long term 5-7 day totals are more on the beneficial side
with recent dry weather. Overall...river flooding does not look
likely. Anticipated rainfall should only result in minor rises.


GRR watches/warnings/advisories...


short term...Meade
long term...Laurens

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