Area Forecast Discussion

FXUS63 KGRR 172029
Area Forecast Discussion
National Weather Service Grand Rapids MI
329 PM EST Sat Feb 17 2018
Synopsis/Short Term/Long Term/Hydro/Climate
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 
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 
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. 
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.
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)
           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)