Meteorological winter ushered in by unseasonably warm conditions

The last week in November has featured a warming trend that is peaking on this first weekend in December.  Though winter doesn't officially begin until December 21, "meteorological winter" (defined by the weather community as December-February) is starting off on a beautiful note with highs reaching 70+ for the next few days!

The cause for the unseasonably warm temperatures can be identified in today's surface and upper level weather maps.  A stationary front is positioned well north of the region, stretching from the NYC area west across the Great Lakes towards low pressure over the Plains. A large ridge of warm high pressure dominates the weather south of that boundary, meaning above normal temperatures can be found throughout the Mid-Atlantic, Southeast, and Southern Plains.

Surface map for Saturday morning showing high pressure along the Carolina coast influencing our weather

Looking up to about 18,000' (or what meteorologists refer to as the 500mb layer, shown below), we see a pattern of "zonal flow" across the nation, which means wind is blowing pretty much west to east.  The 500mb chart below shows that the lines (isobars) in brown are fairly straight from west to east with just subtle peaks and valleys. Wind blows parallel to the isobars, and a zonal pattern means little temperature change and no major storm systems.  The wind at this level is generally the one we watch to see the eventual path of any mid to upper-level storm systems.

500mb (18,000') level of the atmosphere. Zonal (west-to-east) flow indicates no major storm systems are in play across the continental U.S.

In the map below, we are looking up even higher in the sky, about 34,000' (or 250mb) or where the jet stream lives. We also see a fairly zonal flow here with no major ripples in the isobars, other than a moderate trough, or valley, off the U.S. east coast.  In addition, the blue colors, which represent stronger wind and the location of the jet stream, is well to our north, roughly along the U.S./Canadian border.  The jet stream separates warmer continental air from colder Arctic air, so that cold air is being bottled up over Canada.

250mb (34,000') level of the atmosphere valid late Saturday. Zonal flow exists with the jet stream (blue colors) positioned along the U.S./Canadian border, pinning cold air north of the CONUS.

Though the west coast of the U.S. has been pounded by a series of Pacific low pressure systems, the position of the jet stream well to our north will mean the brunt of these systems, as they move across the country, stays to our north, which is important as we look ahead to early next week.

One of these Pacific-origin systems will force the jet stream to begin to take a dip across the middle portion of the country by early in the week.  This will allow a cold front to move into the region on Tuesday, bringing high chances of rain and possibly a few thunderstorms.  This "dip" in the jet stream is reflected in the map below, valid late in the day Tuesday.  You can see the valley that is formed by the black lines that extends roughly down the Mississippi River Valley.

Jet stream map (250mb) valid late Tuesday shows a pronounced trough (or valley) of low pressure over the middle of the country, trailing a cold front moving across the nation's mid-section.

The positive side of the front that moves through on Tuesday, besides some much needed rain, is that since it originated over the Pacific and not in Canada, the air behind it is not all that cold. So while we expect a cool down for mid-week, highs will likely still reach the 60s, which is well above normal for this time of year (normal high is in the mid 50s).

We'll continue to monitor the front that arrives Tuesday and bring you any updates on rain and thunder chances.  For now, severe weather chances look slim to non-existent as the front moves through.  Click here for the complete MWN Forecast.

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