Chesapeake Bay effect snow in North Carolina

Suomi NPP VIIRS Day/Night Band (0.7 µm) and Infrared Window (11.45 µm) images at 0710 UTC, with plots of 07 UTC surface reports [click to enlarge]

Suomi NPP VIIRS Day/Night Band (0.7 µm) and Infrared Window (11.45 µm) images at 0710 UTC, with plots of 07 UTC surface reports [click to enlarge]

Suomi NPP VIIRS Day/Night Band (0.7 µm) and Infrared Window (11.45 µm) images (above) showed a well-defined Chesapeake Bay “streamer” cloud  at 0710 UTC or 3:10 AM local time on 01 January 2018. This cloud feature resulted from the flow of unusually-cold air over the relatively warm water of the bay — a process identical to that which produces the more common “lake effect” cloud bands. With the benefit of ample illumination from a Full Moon, the “visible image at night” capability of the Day/Night Band was vividly illustrated (and a VIIRS instrument on the JPSS series of satellites — including the recently-launched NOAA-20 — will provide similar imagery).

During the subsequent daylight hours, 1-minute Mesoscale Sector GOES-16 (GOES-East) “Red” Visible (0.64 µm) images (below) showed the Chesapeake Bay streamer cloud moving southward. Note that this cloud produced light snow as far south as Currituck, North Carolina (KOTX) from 14-15 UTC or 10-11 AM local time. It is possible that some light snow also occurred across a portion of the Eastern Shore of Virginia and the Outer Banks of North Carolina, but verification is not possible due to the scarcity of surface observation sites in those areas.

1-minute GOES-16

1-minute GOES-16 “Red” Visible (0.64 µm) images, with hourly surface reports plotted in yellow [click to play MP4 animation]

Time series plot of surface weather conditions at Currituck, North Carolina [click to enlarge]

Time series plot of surface weather conditions at Currituck, North Carolina [click to enlarge]

Northwest Mountains Protect Us From the Cold

Much of the U.S. is in the deep freeze, with temperatures in the single digits along the NE U.S. coast and temperatures dropping to -30F or less in Montana, a one day car ride for most of the readers of this blog.

But today, temperatures in western Washington will be relatively balmy, with highs reaching into the lower to mid 40s over western WA and Oregon, while east of the Rockies, the HIGHS will get perhaps to -15F in some locations (see National Weather Service Forecasts for today, Monday):

Why doesn't the cold dense air within the interior of the continent slosh over us, like it is doing over the east coast? 

The reason:  our mountains, which provide  multiple lines of defense that our beloved Seahawks would be jealous of.

Take a look at the minimum temperatures this morning.  In central and eastern Montana, the lows dropped to -30F and colder.   Big Sky Country is frozen solid.  Cross the Rockies into eastern Washington and the situation is relatively balmy, with lows dropping into the teens and low 20s.  Cross another barrier (the Cascades) and drop into western Washington/Oregon and the lows reach only the mid-20s to around 30F.   Cross another barrier (the coastal mountains including the Olympics) and temperatures only dropped into 30s and mid-40sF in some location. 


So we are talking about an 80F difference in low temperatures between the WA coast and central Montana, a distance of roughly 600 miles.

The key to understanding our temperatures situation is the blocking effect of the major regional terrain barriers, coupled with the relatively warm Pacific Ocean.

Currently, cold high pressure dominates the central portions of the U.S. (see NWS sea level pressure map below).  Over our region, there is a weak offshore pressure gradient (high pressure inland), which results in offshore (easterly flow) over the Northwest.   This pattern reduces the impacts of the warm Pacific for the NW interior (click on image to enlarge, the solid lines are lines of constant sea level pressure)


Our terrain is well known to most of you (see map).  The Rockies are a huge and high barrier...it does the heavy lifting of cold-air protection, blocking the coldest and densest air near the surface.   The Cascades are lower and narrower, but do block the coldest air in the Columbia Basin from reaching the wset, with one exception:  in the sea level Columbia Gorge.


But there is a bit more to the blocking effects, which is best understood by a vertical cross section through our regional barriers (see below).   This figure is from my book on Northwest Weather.

As noted earlier, very cold/dense air at low levels over the continental interior is blocked by the Rockies.   Some air (at the crest level of the Rockies) does make it into eastern Washington, but that air sinks down the western slopes of the Rockies and thus is warmed by compression as it loses elevation.  (The air is going from lower pressure aloft to higher pressure near the surface).   The same process occurs with the Cascades and the coastal mountains....each barrier results in warming of the air reaching the surface.

Near the coast, the relatively warm (roughly 50F) waters of the Pacific come into play.  Any onshore flow will flood the coast with mild marine air.

Another way to see the impacts of terrain is from a forecast of sea level pressure (solid lines) and 925 hPa (around 3000 ft) temperatures (white and purple are the coldest temps) for 10 AM toda (see below).  You can see the impacts of the Cascades, keeping the colder air over eastern Washington away from the west.   Look closely and you will notice tongues of cold air and high pressure trying to push across the Cascades...those are the passes.   And look closer (click on image to expand) and you will see a substantial pressure difference across the Cascades, which can drive easterly winds in the passes and the Columbia Gorge.


At Crown Point in the Columbia Gorge, the winds are now gusting to 75 mph, with SUSTAINED winds just under 50 mph. 

With high pressure over and east of our region, there will be plenty of sun on New Years day.

Parade of "Synoptic Debris" Continues into 2018

The past two months have produced remarkable weather in Utah and many portions of the southwest United States. 

Remarkably bad that is, if you are a skier.

Often meteorologists use averaging to illustrate slowly evolving aspects of the upper-level pattern over long periods.  If we do this for November, one sees just a hint of a broad upper-level (500-mb) ridge over the southwest US, with westerly flow strongest just to our north across the US Pacific Northwest. 

Source: NOAA/ESRL
This was a pattern in which there was an active storm track just to our north, resulting in above average precipitation in many high-elevation areas of the Pacific Northwest, but below average precipitation for northern Utah and much of the southwest. 

Source: water.weather.gov
December's pattern was more "high amplitude" with strong ridging along the Pacific coast and deep troughing over eastern North America.
Source: ESRL
This resulted in drier conditions shifting northward into the Pacific Northwest, but "roll over" precipitation helped in portions of the Northwest interior. 

Source: water.weather.gov
The use of average conditions often obscures important aspects of the day-to day variability.  Thus, I put together a long loop of daily dynamic tropopause (jet level) analyses covering November and December.  The one thing to take away from this is how high amplitude (i.e., "wavy") the jet-stream pattern has been across the entire Northern Hemisphere.  We see some troughs moving across the western U.S., but they are often cut-off or strung out.  I call these troughs "synoptic debris."  Sometimes they can produce some snow for us, but this is a pattern that by and large is not one favoring frequent heavy snowfalls in northern Utah.  


Which brings us to the forecast.  Basically, more synoptic debris for western North America during the first 10 days of 2018, as advertised by the GFS. 


The next four days look dry in northern Utah, and then a piece of that synoptic debris, flirts with the area.  Some ensemble members give us some precipitation in the central Wasatch (Alta-Collins depicted below), although there are some members that keep snowfall totals to a minimum. 

Basically, this is business as usual for this winter.  Keep expectations low and hope for the best.  I'm debating whether or not to bring my rock skis in for a much needed tuneup, or save the money and just keep beating the hell out of them.