Feb. 11-12 2019 Wintry Mix Event

aiannone wrote:18z NAM has prolonged ZR

That’s actually sleet. The deeper shade toward southwestern PA is actually the ZR and they can keep it lol

I think some folks may get 4”+ just on Sunday night into Monday. Could be 195 On south. This will probably have some surprises. One thing is becoming clearer surface temperatures are going to be slow to rise.

Frank is in Spain. I have to wake my son up for a Futsal game and then Im goig to the Islander game later so I cant do as detailed of an alysis as Id want but take a quick look at the two images below.

First is the surface as the moisture is just about to move into the area. Nice HP, avging 1039-1041 on modeling) parked to our N. Honestly cannot complain here. Maybe Id shift the center S just a tad but really its not bad. Butlook at the HP in western Canada. Not as well defined and weaker.



Here is the 500mb anomaly map. Notice the EPO region. Unfort there is a negative(cool colors) that is splitting Alaska in half. As you can see there is some ridging/blocking extending into part of the EPO region but honestly I would have liked to see it extend further N as indicated by the yellow dashed line. This would have created a much stronger push of cold from the N. Instead the lack of reinforcing cold air, strong HP, behind our nice HP parked to our N is a weakness on the western flank of said HP to our N. Because the S energy is so strong it allows our primary to find this weakness and cut well to the west, but it also serves to allow the primargy energy to scour out the cold with less resistance to the N. Prev modeling had a stronger Alaska ridge and a secondary popping of the Delmarva tarcking S&E of LI which would aid in keeping the lower levels with a more NEasterly wind flow. The secondary now appears to track over us instead which allows the easterly flow to develop sooner. The CAD should still be strong enough for all snow for everyone at the start.






Hree is uptons discussion and honestly I couldn't have said it better myself.


Focus of attention this period will be at the very onset as the
first in a series of southern branch storm systems impact the
region. The first of which poses a winter weather hazard as
a strong polar high (1040mb) passes to the north across eastern
Canada and the northeast allowing cold air to dam east of the
Appalachians.

A primary low will will move through the Great Lakes Region with a
secondary low developing in the Mid-Atlantic Region. The average
model track of this secondary low continues to trend west and is
expected to pass over us Tuesday night. This trend results in an
increasing likelihood that an elevated warm layer eventually changes
PCPN to rain for the entire forecast area, however there will be
areas across the northern tier that may never have surface
temperatures rising above freezing. The greatest uncertainty with
this forecast resides on the front end of the storm and how quickly
the transition from all snow to a mix, then to rain occurs. Although
all models show plenty of warm air aloft for transitioning, there
are significant differences on how quickly this occurs - with the
NAM being the quickest, ECMWF the slowest, and GFS in between. Due
to the anticipated track of the storm, leaned towards the warmer
side of the average of the 3 models. Small timing differences could
have a significant impact on snow/ice totals, thus will not issue
any watches or advisories at this time. This is mainly a 5th-6th
period event and would like to see if better agreement arrives with
the 12z suite of models.

Currently have snow/sleet accumulations of 4-6 inches NW of the city
and 2-4 inches at most other spots except eastern LI where 2 inches
or less is anticipated. Regarding ice accumulations, the greatest
uncertainty is for areas north and west of I-95 where marginal
surface temperatures could spell more ice accretion than currently
forecast, which is currently zero for more than half of this area.
Where accretions are currently forecast, looking at mostly up to a
tenth or two of ice, but this might be underdone as well.

These boundary layer storms always tough calls. The first wave is all snow and could drop a few inches.
EPA I like 3-5” all snow, 1.5” sleet/graupel and .25 zr. Then ending as rain. Absolute potpourri of precipitation types probably. One thing is surface temperature will be well below freezing for good portion of event so whatever falls accumulates.

In this winter that’d be as significant as seen since November.

heehaw453 wrote:These boundary layer storms always tough calls. The first wave is all snow and could drop a few inches.
EPA I like 3-5” all snow, 1.5” sleet/graupel and .25 zr.   Then ending as rain.  Absolute potpourri of precipitation types probably.  One thing is surface temperature will be well below freezing for good portion of event so whatever falls accumulates.

In this winter that’d be as significant as seen since November.

Yeah details with these types of storm often need now cast.

billg315 wrote:Heehaw and Scott beat me to the punch. Looking at the 6z runs I’d say throw out the snow/ice projections and focus on temps. There is no way the models can nail exact amounts on a system like this because the dynamics may be very localized. The 6z runs all show the cold air at the surface not budging for most of Tuesday. Many areas don’t get above freezing until after 3 or 4 pm - if then. Even at the 850 level the 0* line stays south of most of the area until afternoon.

This leads me to believe this will be far more snow sleet and freezing rain than just plain rain (which will come, but not until evening). The key to accumulation is exact timing of changeovers and intensity during those times. If the snow lasts just a couple hours longer somewhere, and it gets heavy during those couple hours, that’s a difference between maybe 2” vs 4 or 5”. If the sleet lasts four or five hours, while it doesn’t accumulate as fast as snow, it will accumulate. And all you need is an hour or two of freezing rain for all sorts of problems.
I think this will actually be a very disruptive storm Tuesday despite what may look like paltry snow totals to some. And I don’t think we’ll even get a fix on those totals until it’s happening.
The focus of this system should be more on the problems it can cause than pure amounts because it hits before AM rush and may actually continue almost into the PM rush for many before you see any non-frozen rain.

Very well said. I totally agree with your point about a disruptive storm. Surface temperatures mean a lot in that regard.