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Area forecast discussion 
National Weather Service Grand Rapids Michigan
327 PM EDT Tuesday Jul 22 2014

Latest update...
synopsis/short term/long term/Hydro/marine

Synopsis...
issued at 327 PM EDT Tuesday Jul 22 2014

A frontal system will push through the area this evening and during
the early overnight hours tonight. This will bring a good threat of
showers and thunderstorms...some of which could become severe. The
best chance for severe weather will be along and west of U.S.-131.

Showers and storms will have moved out of the area by daybreak
Wednesday morning and clouds will clear out. Much cooler air will
move in for the second half of the work week along with drier
conditions. The next chance for rain will come this weekend with the
next system expected to affect the area.

&&

Short term...(this evening through Thursday night)
issued at 327 PM EDT Tuesday Jul 22 2014

The main concern in the short term remains focused on the convection
expected this evening into the early overnight period.

No changes to the thinking of possible severe weather this evening.
Diurnal instability is building this afternoon with ml convective available potential energy around
or over 1000 j/kg as of 18z this afternoon. We continue to expect
however that a strong cap around 800-850 mb will hold off convection
through at least middle afternoon...even as moisture transport ramps up
over the area. Also limiting convection chances this afternoon is
mesoscale ridging just downwind of Lake Michigan.

We will be waiting for convection to fire ahead of the surface front now
moving into eastern Upper Michigan and the Green Bay area as evidenced by the
enhanced area of cumulus per visible satellite imagery. It is near the front
that the cap erodes and the leading edge of the best instability
across WI moves in across western Michigan this evening. We will be looking at
1500-2000 j/kg of ml cape to be in place /highest west of U.S.-131/
as the front...and a short wave moving toward the area from SW Minnesota
moves in. This instability will be an elevated mixed layer based a
little bit above of the surface. This scenario means that even as we
cool off right at the surface some tonight after sunset...the
instability will remain aloft. It also means that Lake Michigan
should not have as much of an influence as if convection was purely
surface based.

Severe weather remains a distinct possibility this evening. The
frontal passage...short wave approaching...and elevated instability
all seem to coincide with some decent deep layer and low level shear
this evening. Deep layer shear values are around 25-35 knots...and
0-3km shear values of 20-30 knots are present with the low level jet
around...would be sufficient for an organized wind threat. Large
hail will be possible with fairly healthy cape in the -10 to -30c
layer. We continue to think an isolated tornado threat exists with
0-1km shear vales of 20-25 knots and 0-1km helicity values of around
100-150 m2/s2. Low inversion heights will allow stronger convective
winds to have a better chance to reach the surface. Heavy downpours will
also be a threat with high precipitable waters in place.

The severe threat looks to diminish by 06z as the low level jet moves east of
the area. Instability diminishes with the moisture transport moving
out and as the front moves through. We should see all precipitation end by
09-12z Wednesday as the short wave pushes through.

The remainder of the short term through Thursday night then looks to be
quiet and relatively cool. We will see a short wave drop through in
the northwest flow aloft on Wednesday night. This wave will have no moisture to
work with with a dry nearly low level flow in place and broad scale
subsidence in place being downwind of the long wave ridge. Temperatures on
Wednesday and Thursday will generally be in the middle to upper 70s with h850
temperatures around 9-12c.

Long term...(friday through tuesday)
issued at 327 PM EDT Tuesday Jul 22 2014

Timing of chances for rain (showers and storms) this weekend is the
main challenge in the long term period. The upper pattern switches
from somewhat of a flat west-northwest flow on Friday to a deep
upper trough over the eastern half of the country by next Tuesday.
Details in how we get from the west-northwest flow to the deep upper
trough are where the issues lie with this forecast.

The differences pertain to the lead shortwave that digs into the
Great Lakes and aides in deepening the upper trough. The GFS is
quicker bringing the shortwave in Saturday...where as the European model (ecmwf)
holds the lead wave off until Sunday night. Both models get to the
same point by Tuesday...but the weekend details are what is in
question. Confidence is a bit lower due to the discrepancies in
models. Tend to favor the slower European model (ecmwf) progression...but not certain
in this evolution. The lead wave that deepens the upper trough is
currently seen spinning over the British Columbia coast. At this
point it is almost stationary.

As for the details have chance probability of precipitation in place over the weekend to
take a middle Road between the wetter/quicker GFS and the
slower/drier European model (ecmwf). The pattern looks showery into early next week
as the upper trough settles in. Highs around 80 this
weekend...sliding back into the 70s for early next week.

&&

Aviation...(for the 18z tafs through 18z Wednesday afternoon)
issued at 154 PM EDT Tuesday Jul 22 2014

VFR conditions in place area wide at 18z...which will continue
through the remainder of the afternoon. The focus is on the
extent of thunderstorm development this evening. Essentially
expecting scattered thunderstorms to develop and sink south through
the taf sites between 22z and 06z. Best chance for storms will be
at kmkg...kgrr and kazo...with slightly lesser chances inland. At
this point not confident enough to go prevailing storms at any
given location so used thunderstorms in the vicinity wording through the evening time
frame. Some storms may contain strong gusty winds in excess of 35
knots.

Overnight...the threat exists for a MVFR cloud deck to settle into
the taf sites on northerly winds behind a cold front. At this
point used scattered wording at 1500ft to convey the chance of
this deck developing. A chance at some MVFR fog for a time
overnight as well with high dew point air in place. Skies should
gradually clear/scatter out on Wednesday as drier air works in
from the north. Some models indicate a MVFR deck holding on across
much of the area though through Wednesday morning.

&&

Marine...
issued at 327 PM EDT Tuesday Jul 22 2014

As expected...waves continue to ramp up. The Ludington buoy is now
at 5.2 feet with stiff south winds. Port Sheldon is at 3.3 feet.
So...the ongoing south flow event looks to be handled well by the
current headlines (small craft/beach hazards in effect) northern 3
zones.

The flow weakens briefly tonight...before switching north and
ramping up once again. Have all areas covered by a Small Craft Advisory/bhs late
tonight and Wednesday. Expecting waves pushing into the 4-7 foot
range in the southern zones in a long northerly fetch.

In terms of beach impact today/S event in the north half of the
marine area may be the higher threat given the warm temperatures...and
therefore a larger beach population. Tomorrow/S waves will be
bigger but the impact may be mitigated by cooler beach air
temperatures...resulting in less folks on the beach and in the
water.

&&

Hydrology...
issued at 327 PM EDT Tuesday Jul 22 2014

Still expecting a fairly quick progression of the convection into
late tonight/early Wednesday. As such...rainfall amounts should not
get out of hand (precluding a flood threat) but still with precipitable water values
reaching the 1.75"-2.00" range there could be heavy downpours and
localized ponding on roads in the heaviest storms. Overall amounts
should be a quarter to half inch with heavier storms producing
locally more than that. With respect to area rivers and
streams...would not expect any significant rises to occur with this
activity. Rivers are all well within banks and it is unlikely that
any would get close to bankfull with this type of setup.

&&

GRR watches/warnings/advisories...
Michigan...Beach hazards statement from 2 am EDT Wednesday through
Thursday morning for miz056-064-071.

Beach hazards statement through late Wednesday night for miz037-
043-050.

Lm...Small Craft Advisory until 2 am EDT Thursday for lmz847>849.

Small Craft Advisory from 2 am Wednesday to 8 am EDT Thursday
for lmz844>846.

&&

$$

Synopsis...njj
short term...njj
long term...Duke
aviation...Duke
hydrology...Hoving
marine...Duke

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