Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Pittsburgh PA
245 PM EST Friday Mar 6 2015
clear skies will gradually be replaced by clouds through Saturday
as cold air slowly erodes this weekend. More seasonable
temperatures look to finally return by midweek.
Near term /through Saturday/...
middle and high clouds have begun to increase across the area as an
exceedingly weak...moisture starved...and essentially surface
disconnected system is tracking through the region. With little
increase in low level moisture...no precipitation is expected from
this system. The main impacts will be a slow increase in
southwesterly flow as surface high pressure moves off to the
east...as well as the aforementioned middle and high cloud increase.
The effects of this will basically be to hold back temperatures
through sunset...with little more fanfare than that.
Going into tonight...the gradient flow from the southwest should
increase just a bit...although the preponderance of guidance
suggests our boundary layer may decouple from the West Virginia
Panhandle eastward before this occurs. Additionally...some
clearing may be possible for an interlude between the weak system
this evening and another northern stream wave that approaches for
Saturday afternoon. Decoupling combined with come clearing should
allow for many areas to drop a fair bit tonight...even as MOS
guidance fails to show much diurnal fall at all. That said...lows
were forecast combining the colder end of guidance...and should
clearing materialize in a more widespread fashion...single digits
may again be possible.
By Saturday...clouds will again be on the increase. While model
solutions really have failed to trend much stronger with the
northern stream wave...they are managing to kick up layer moisture
a bit. Additionally...fairly unstable low level lapse rates seem
to be generated...and the majority of the lift in time height
sections seems to be working on the unstable portion of the
profile as the system passes. Because the majority of the lift
traverses to our north...probability of precipitation were generally built to favor the
I-80 corridor and taper southward...with moisture likely being too
lean to squeeze out anything south of Pittsburgh.
Soundings from both the NAM and GFS suggest that wet bulbing
should keep any precipitation that falls on Saturday afternoon as
snow. This seems likely even as warm advection and efficient
mixing ahead of the system may well warm virtually the entire County Warning Area
above freezing by afternoon. In fact...across the far southern
County Warning Area...even a 40 degree reading would not be too shocking. This is
a far cry from the below zero readings we had this morning...and
downright tropical compared to what February was like. It
certainly seems like old man winter may be having his back broken
in our area very shortly. Fries
Short term /Saturday night through Monday/...
synoptic pattern in the short term begins with northwest flow
aloft...and multiple weak shortwave troughs passing through the
Great Lakes. This will keep temperatures relegated to just below
average through the weekend and at least slight chance probability of precipitation in
along the north.
Deeper middle-level wave passing late Saturday night into Sunday
morning will reinforce low-level moisture pooling south of Lake
Erie and dammed up against the higher terrain to our east.
Meanwhile...moisture above 700mb is transient and moves quickly
east of the area. Thinking that really light snow will be the main
precipitation type. But...with the bulk of the moisture confined
to the low-levels and the top of that moisture around -8c...the
potential for freezing drizzle exists. Still some uncertainty
surrounding extent of moisture and timing...so confidence not high
enough to include freezing drizzle in the grids. Will need to
continue to monitor this with future runs of higher-res models.
Flow begins to go more zonal towards the beginning of next
week...allowing temperatures to moderate under mostly dry
Long term /Monday night through Friday/...
the extended period looks to be relatively quiet as zonal flow
sets up across the forecast area and we see a gradual warming
trend in temperatures...extending slightly above normal by
Aviation /20z Friday through Wednesday/...
middle and high clouds in association with a weak system will move
through the area this afternoon. A brief break before MVFR ceilings
with some light snow showers across the northern sites will be
possible by afternoon. Winds will increase slowly from the
southwest and become gusty by Saturday afternoon.
Outlook.../Friday night through Tuesday/...
restrictions are possible through the weekend.