Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 172029
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
329 PM EST Sat Feb 17 2018

LATEST UPDATE...
Synopsis/Short Term/Long Term/Hydro/Climate

.SYNOPSIS...
Issued at 329 PM EST Sat Feb 17 2018

A short period of light snow will fall across most of the area late
this afternoon and early this evening. Most areas will see an inch
or less fall before ending by midnight.

A warm and wet period will develop starting early Monday morning,
and will last into Wednesday. Periods of rain can be expected with a
few thunderstorms possible. Some heavy rain will be likely.
Temperatures could approach 60 degrees across the southern areas
from Monday night into early Wednesday with cooler temperatures up
north.

Cooler air will filter in for the end of the week. Generally drier
conditions will be likely for the end of next week, with
precipitation potentially moving in again next weekend.

&&

.SHORT TERM...(This evening through Monday night)
Issued at 329 PM EST Sat Feb 17 2018

Our biggest concerns in the short term are looking at the potential
for mixed pcpn up north late Sun night and then rain/thunder trends
for Mon and Mon night. One change we have made to the fcst is
pulling out the mixed pcpn for Sun night, as temps should warm above
freezing before pcpn moves in up there.

No real changes for the light snow event coming through this
evening. We continue to expect light snow to accumulate up to an
inch over western areas with about 3-6 hours of light snow. We can
not rule out some rain briefly mixing in across the south where
enough warmth from diurnal heating will help out.

We will see a dry period from overnight tonight through the early
overnight hours on Sun night. Sun might even be a decent day with
clouds expected to clear out a bit, and highs sneaking up into the
lower 40s.

Pcpn will make a move into the area from south to north later Sun
night and Mon morning, pretty much all in the form of liquid. This
rain will be the result of the upper pattern shifting to a SW-NE
orientation over the region as strong upper troughing digs south
across the Western U.S. coast. We will see the initial wave approach
late Sun night, and draw Gulf moisture overhead with a 60 knot low
level jet pushing in.

Rain will make it roughly to around I-96 by 12z Mon, and then
progress north through the remainder of the area Mon morning. This
is important as the models have been fairly consistent in showing
temps near freezing up north Sun night. There is very good
confidence that pcpn will not make it there before temps warm above
freezing, so mixed pcpn potential is quite low.

Once the rain overspreads the area, it will continue intermittently
with multiple waves moving along the nearly stalled out frontal
boundary parallel with the upper flow overhead through Mon night and
into the long term. Instability parameters indicate that we will see
sufficient elevated instability for thunder move in Mon afternoon
and continue through Mon night and beyond.

.LONG TERM...(Tuesday through Saturday)
Issued at 329 PM EST Sat Feb 17 2018

Periods of rain are expected to continue into the long term period
with a deep southwest flow in place aloft. At the surface, southerly
winds with a flow off the gulf will persist both Tuesday and Tuesday
night. We expect widespread light to moderate rains that will become
occasionally heavy at times from Tuesday into Tuesday night and
perhaps into Wednesday morning. Thunder chances exist Tuesday and
Tuesday evening with 850mb LIs near zero. Some weak CAPE is seen as
well.

The setup looks about the same as yesterday with a frontal boundary
draped southwest to northeast across the area. Waves of low pressure
are forecast to ride northeast along the front and enhance rainfall.
The last wave looks to move through Tuesday evening. Precipitable
water values have increased in the model runs today and are now near
1.4 inches via the GFS. This would be record level PWAT values for
February based on DTX sounding climatology. 850mb moisture transport
remains strong Tuesday into Tuesday night especially across Southern
Lower Michigan.

Some light snow will mix in before the precipitation ends Tuesday
night into Wednesday on the back edge of the system. We are not
expecting much in the way of accumulation however. Mainly dry
weather is expected Wednesday night into Thursday night as high
pressure drifts through the region.

Our next weather maker is associated with a shortwave aloft moving
through the Great Lakes region Friday into Saturday. Model
differences exist with this system, but we tend to lean towards the
GFS/Canadian solutions that are quicker and more amplified. Have
both light rain and light snow in the forecast during this time
frame as temperatures will be varying within 10 degrees of the
freezing mark. Precipitation amounts look to be a half inch or less,
so not expecting big impacts.


&&

.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
Issued at 100 PM EST Sat Feb 17 2018

A quick burst of snow will move in from the west this evening,
generally after about 22z. Between 22z and 03z, snow will move
through the TAF sites with conditions dipping into the MVFR
category (both ceilings and visibilities). Conditions will improve
late tonight back to VFR once again between 09z and 12z. VFR
weather will then prevail into midday on Sunday.


&&

.HYDROLOGY...
Issued at 329 PM EST Sat Feb 17 2018

Confidence is increasing in 1 to 2 inches of rain with narrower
swaths of 3 inches possible Monday through Wednesday morning. River
levels are running above normal for this time of year in the upper
Grand basin (greater Lansing area), with a couple already near
bankfull. Snow water content on an area-wide average is estimated
around a half inch.

The rain and snowmelt is expected to cause most rivers to rise out
of their banks by Tue in headwaters and later in the week in
downstream main stems. Familiar nuisance flooding would be expected.
Some rivers may reach minor flood stage which is a little less
typical and starts impacting roads and some structures. A comparable
flood in recent memory occurred in April 2017. Right now we do not
expect most rivers to surpass what was seen in that event, although
smaller basins that receive bursts of heavier rain can quickly rise
higher than expected. Ice on the rivers may break apart then jam up
downstream, resulting in unpredictable fluctuations in levels.

How rare is this rain event? Two-day precip totals over 2 inches
during February have only occurred a handful of times in 120-plus
years of records at Muskegon, Grand Rapids, and Lansing. Most
recently it occurred in 1997 at GR and Lan, and 2001 at Mkg. The
records for two-day Feb precip amounts are 2.41 inches at Mkg in
2001, 3.56 in 1997 at GR, and 4.40 in 1938 at Lan.

Note: Current river level forecasts published on AHPS do not account
for precipitation expectations beyond 48 hours.

&&

.CLIMATE...
Issued at 329 PM EST Sat Feb 17 2018

With the warm and wet conditions looking more likely, we looked at
the high temp records, max low records, and precipitation records
for Mon, Tue, and Wed.

                 Mon 2/19        Tue 2/20       Wed 2/21

Muskegon   High  58 (2017)      62 (2017)      58 (2017,1930)
        Max Low  44 (1994)      40 (1953)      41 (2017)
           Pcpn 0.88"(1994)    1.13"(2002)    1.44"(1997)

Grand Rapids
           High  62 (2017)      61 (1930)      66 (1930)
        Max Low  47 (1994)      50 (1930)      47 (1930)
           Pcpn 0.85"(1939)    1.40"(1898)    2.96"(1997)

Lansing
           High  62 (2017)   59 (2017,`83,`30) 62 (1930)
        Max Low  42 (1994)      42 (1930)      42 (2017)
           Pcpn  1.49"(1939)   1.04"(1891)    2.14"(1997)

&&

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

$$

SYNOPSIS...NJJ
SHORT TERM...NJJ
LONG TERM...Duke
AVIATION...Duke
HYDROLOGY...CAS
CLIMATE...NJJ



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