Rio Grande Valley:
Merry Christmas to all! Pleasant weather today for the RGV with the cold front passing through the entire Valley around midnight. High temperatures actually reached 68 in McAllen, 67 in Brownsville, and 66 in Harlingen (a little warmer than expected). Winds were out of the northwest around 20 mph, with gusts up to 25 mph. A Freeze Warning is in effect for the entire Rio Grande Valley from 2 am to 9 am tomorrow morning. Clear skies tonight will allow cold air in the atmosphere to sink to the surface. Lows will be in the low 30s and even in the upper 20s in the extreme northern Valley, with winds out of the north northeast at 5-10 mph.
(12/26): Sunday will start off cold and will warm up to the mid 60s under sunny skies. Early windchill values will be in the mid 20s, so make sure you bundle up if going outside during the early morning hours. Winds will be out of the northeast at 10 mph. Sunday night looks to be chilly, but not as cold. Lows will range from upper 30s in the upper Valley to lower 40s in the lower Valley under mostly clear skies. Winds will be out of the northeast at 5-10 mph.
(12/27): Starting Monday, mid-level troughs will dig across western and central U.S., shifting the winds from the northeast to the southeast at the surface and bringing in moist air from the Gulf of Mexico. Thus, this wind shift will also bring in cloud cover over the Valley as well as increase precipitation chances for the RGV during the evening. Highs will be in the mid-to-upper 60s. Lows will be in the mid 50s with a 30% chance of showers.
(12/28): A slightly better chance of rain is in store for Tuesday(40%), but will be light if any falls. Afternoon highs will reach lower 70s with mostly cloudy to overcast skies. Breezy winds will be out of the southeast between 10-15 mph, with gusts up to 25 mph. Tuesday night, slight chance of rain(30%) with lows in the upper 50s/lower 60s.
Rest of the Nation:
A cold front off the western coast is generating rain and snow, again, for California as well as for Oregon and Washington. Winter storm warnings and advisories have been issued for the Sierra Nevada region. This cold front is associated with an upper-level trough at 500 and 700 millibars (mb). In the upper-atmosphere, positive vorticity advection (PVA) generally occurs on the right side of a trough (or left side of a ridge). PVA enhances lift at the surface, creating clouds and precipitation most of the time. This is why there is precipitation falling over California at this time. All levels in the atmosphere support precipitation. 500 mb map from a computer model (18Z GFS) is shown below. Click on the picture to enlarge.
From the graphic above, there is also a lot of positive vorticity advection for the east coast. Numerous winter storm warnings and advisories have been issued for most of the east coast. Snow has been falling in Atlanta, GA and other areas in the southeast for most of the day because of the this upper-level trough and surface low, located south of the Florida panhandle. This is the same surface low that moved through Houston and had a cold front that came through the Rio Grande Valley last night.
Having talked about this low, it is expected to move east and enter the Atlantic Ocean and then move north, developing into a major nor’easter that may give the New England states a major blizzard in the next 24 to 36 hours! Rough waves, hurricane force winds and lots of heavy snow will be a major threat to those who are living on the east coast, and especially those living in the northeast. I wouldn’t be surprised if some isolated areas get 2+ feet of snow from this system. Below is a graphic from a forecast model 30 hours out. Click the picture to enhance quality.