Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Grand Rapids Michigan
issued by National Weather Service Detroit/Pontiac Michigan
315 PM EDT Wednesday Apr 1 2015

Latest update...
short term...long term...and marine

Synopsis...
issued at 327 am EDT Wednesday Apr 1 2015

A period of rain and potentially thunderstorms is expected early
Thursday morning through Thursday evening as a cold front sweeps
through lower Michigan. Moderate rainfall and gusty winds at times
will be possible with storms on Thursday. Cooler air filters back in
Thursday night behind the cold front. Rain and snow chances will
return Friday into Friday night. Highs in the 40s to around 50
degrees are expected Friday through Sunday.

&&

Short term...(this evening and tonight)
issued at 315 PM EDT Wednesday Apr 1 2015

A residual trough draped across southern Michigan lead to chaotic
wind/thermal patterns through the early afternoon. Temperatures
north of a line from about Lansing to Pontiac were slow to rise
through the morning with winds staying light and variable. Recent
observation show this trough has almost completely filled as temperatures are now
jumping into the 50s across most of southern Michigan as southerly winds
have finally spread northward into middle Michigan.

Heading into this evening...a warm front to our west will extend
into the region as it lifts northward. This front should be through
mbs around 06z. A Stout 50 knot ll jet between 900-950mb will
initiate a period of decent warm air advection in the layer which will then result
in a steep inversion as bl cools. The SW gradient will increase
overnight as the warm sector moves into southern Michigan. Winds will
increase overnight while becoming southerly. The column is pretty
dry with only around a half inch of precipitable water ahead of the warm front.
This will lead to a dry frontal passage with the precipitation remaining to
the north tied closer to the parent low and the pressure falls and
forcing just ahead of it. Middle level moisture advection and
isentropic ascent along the front will increase the middle cloud
presence mainly across middle Michigan through the night. Thermally...the
front will likely lead to a non-typical nocturnal cooling trend with
lowest temperatures occurring early in the night with steady warming toward
morning as warm air advection increases and clouds move in. This should hold low
temperatures in the 40s for most locations.

&&

Long term...(thursday through wednesday)
issued at 315 PM EDT Wednesday Apr 1 2015

Convectively induced shortwave currently spawning convection in
southwest Missouri will lift north overnight as deeper southwest
flow spreads across the central Continental U.S. And Great Lakes. Arrival will
be roughly coincident with the low-level warm front and is expected
to support at least a partial coverage of showers through middle-
morning. A rumble of thunder is not out of the question as morning
showers will be most likely remain tied to the lead edge of the
moisture transport axis.

General agreement among NAM/GFS/ECMWF that strong right entrance
dynamics juxtaposed within the favorable region for supergeostrophic
acceleration and divergence aloft will occur in vicinity of northern lower
Michigan. Cross-section analysis reveals a strong tilting component
resulting in a considerable fgen response aloft while the baroclinic
zone below 850mb remains essentially stationary, tending only to
contract with time, 15-21z. The strongest response is prognosticated by the
NAM/GFS to align roughly along a Muskegon to Saginaw Valley line
while the European model (ecmwf) is slightly further south and east, a nod toward the
potential for a more progressive outcome.

850-700mb wind maximum is forecast is forecast by the 12z NAM to emerge
out of Illinois and spread north into central lower facilitating
excellent boundary layer moisture transport along the frontal zone.
With 850 mb dewpoints nearing 10c and developing low density cape, the
stage is set for moderate to perhaps locally heavy rainfall with a
bit of embedded thunder persisting through midday within the
aforementioned favored axis. The progressive nature of the upper
flow and relatively shallow (<10kft) warm cloud layer will seemingly
prevent any threat of a formidable heavy rain episode.

Approaching convectively enhanced shortwave with origins over the
plains will then race across Michigan during the aftn, forcing the front
through the state by evening as jet support rapidly peels northeast.
Confidence is moderately high in a band of 0.4-0.6" quantitative precipitation forecast before the
front accelerates southeast, but would not be terribly surprised to
see locally higher amounts. It is here that it is Worth noting the
slightly more progressive nature of the European model (ecmwf) which would result
favor a more smoothed quantitative precipitation forecast field and a lower potential maxima. In
general, expect lower totals further south and east given decreasing
forcing and forward-accelerating cold front, though 0-3km MUCAPE
near 500 j/kg may be sufficient to produce a couple of thunderstorms
capable of producing brief heavy downpours by afternoon. High
temperatures in advance of the front and ahead of any more
persistent showers/thunderstorms will push into the 60s as winds gust 25
to 30 kts.

Resident 850mb thermal field will cool back into the low single
digits by Friday morning as the Theta-E axis folds south and east
late Thursday into Thursday night. Baroclinic zone associated with
the approaching trough will then slide through on Friday,
substantially cooling the column by late Friday evening. Trailing
energy along the northern periphery of an emerging jet streak will
force a frontal wave to develop along this boundary. Strong coupling
of the upper jet structure is prognosticated by the ECMWF, with peak deep
layer forcing transiting lower Michigan roughly 18-00z. Taken at
face value, European model (ecmwf) suggests the column will be cold enough for all
snow with the exception of the near-surface layer. Light
accumulation along the edge of the precipitation shield. No
significant changes needed to this portion of the forecast.

The weekend will start off dry as high pressure nudges into the
region on Saturday. Increasing clouds are expected through Saturday
as the next system approaches the Great Lakes region. This system
will bring rain and snow showers to Southeast Michigan through
Sunday. Low temperatures on Saturday night will be in the upper 20s
to low 30s with temperatures quickly rising after sunrise into the
middle 40s by Sunday afternoon. Therefore...the best chance for any
snow will be Saturday night and mainly north of the M-59
corridor...however...no accumulation is expected at this time. The
next major low pressure system arrives early next week. There is
uncertainty about the track and timing of this system though...with
some models bringing in this system by Monday morning and others not
bringing it in until Monday night.

&&

Aviation...(for the 18z tafs through 18z thursday)
issued at 100 PM EDT Wednesday Apr 1 2015

Relatively light winds and no restrictions due to clouds will
continue into the evening. Winds will turn to southerly everywhere
by sunset and start to steadily increase and gain a gust component.
Solid VFR ceilings will start to establish around sunrise with
showers and possible thunder quickly on its heals through the
morning hours.

&&

Marine...
issued at 315 PM EDT Wednesday Apr 1 2015

Southerly winds will increase through the evening hours ahead of an
approaching cold front. Small Craft Advisory conditions are expect
through the night and much of Thursday. Showers and a few
thunderstorms will sweep across the region Thursday morning and
early afternoon.

&&

Hydrology...
issued at 1120 am EDT Tuesday Mar 31 2015

Rivers continue to remain well below advisory levels with only small
and momentary responses noted from recent precipitation. Northern
sections of the forecast area will see slower responses due to
snow melt with last nights snowfall.

We will see a rather active pattern set up through the next 7
days. Much of this comes in two waves...one Thursday then the
second very late in the weekend and into the next work week. General
amounts on total precipitation through the period are around one
inch or so. This is fairly evenly divided between the two waves
which should be favorable for response times. No flooding is
expected at this time. However...the heavier rain that is
anticipated to our south through the Ohio Valley should be watched
in the event that any adjustments are needed in precipitation
amounts.

&&

GRR watches/warnings/advisories...
Michigan...none.
Lm...Small Craft Advisory from 8 PM this evening to 2 PM EDT Thursday
for lmz844>849.

&&

$$

Synopsis...ebw
short term...dk
long term...jvc/rk
aviation...Mann
hydrology...jam
marine...Mann

National Weather Service Glossary of Abbreviations