UPDATE: THU JAN 21 - 8:30 AM. The period from Tuesday 1/27 to Thursday 1/29 is likely to feature several rounds of icy precipitation for the Mid-Atlantic similar to the February 2007 ice storms. This morning's update is to clarify timing and impacts of this event as well provide a scientific rationale for my outlook next week. For administrators and school officials, the good news is Monday should be dry with little chance for precipitation. However for the Baltimore County school community, there is the distinct possibility of an extended weekend if the first round of winter weather arrives Tuesday as expected. The specific dates at risk are Tue 1/27 and Wed 1/28. For most of Maryland, I predict this will be more sleet/freezing rain than heavy snow. I really hope the atmosphere does it's part in making the decision easy for school districts, (as in an early morning onset of precipitation) so we can all avoid repeating the "Valentine's Week Massacre" of 2007.
RATIONALE: Above is a 500 millibar "heights and vorticity" map for next Tuesday 1/27 showing the upper level flow and where the air is spinning. The current conundrum is how computer models and forecasters alike are resolving the "cutoff low" projected to be off California. As you can see, the downstream flow toward the Mid-Atlantic is fairly tight and from the west-northwest. This indicates the pattern can deliver a supply of cold air masses from that direction. Medium range forecasts show strong High pressure cells pressing southeast from central Canada starting this weekend. Meanwhile, a moisture laden open wave arrives on the west coast. This is best depicted in the CPC US Hazards Assessment. As the upper level ridge over western Canada breaks down (evidenced in the PNA index), and the Madden-Julian Oscillation (the MJO) moves into phase 1 (I'll explain that one later), we have the right environment for pieces of energy from the 500 mb cutoff low to be ejected eastward across the rockies. HPC is watching this closely, although the Sterling VA NWS is not buying into these ideas yet, at least not publicly. What's clear is the European model signaling that a series of strong short-waves could be released into the flow, and after crossing the southern plains, will encounter an environment favorable for tapping Gulf moisture on their way to the Mid-Atlantic. With a cold dome of high pressure in place by Monday, it's now easy to see why winter weather will be "coming around again." Per HPC's 7:50 am 1/22 discussion:
A CONFLUENT MID LEVEL FLOW PATTERN EAST OF THE ROCKIES IS A BAROCLINIC BREEDING GROUND FOR WEAK SYNOPTIC WAVES JUICED BY THE GULF OF MEXICO...WITH THE ECMWF AND ENS MEANS OF BOTH GFS AND ECMWF INDICATING AT LEAST ONE WIDE SWATH OF RAIN...ICE...AND SNOW FROM THE LOWER RED RIVER VALLEY EAST AND NEWD INTO THE MID ATLANTIC REGION DAYS 5 AND 6 TUES/WED.
PREVIOUS INTRODUCTION: WED JAN 21. The map below posted earlier forecasts for next Monday 5000 foot temperatures along the Mid-Atlantic to be -16 to -18 C, approximately 0 degrees F... plenty cold for snow or sleet to form and reach the surface intact. Early indications of available moisture show a liquid equivalent of approximately 1.00 inches could fall across the region area between Monday and Thursday of next week.
Expect your weather outlets to downplay this situation for several more days, and they are right to do so, as the long range is always uncertain. However, I am hanging my hat on the mesoscale physics of the situation, by saying that if a significant supply of moisture arrives on the California coast, it HAS to go somewhere. Teleconnections (NAO, PNA) and the current 500 mb upper level pattern both suggest that energy has no choice but to head east toward approaching Arctic high pressure. The potential of this scenario is already being monitored in the HPC extended discussions as well as by the Philly/Mount Holly NWS. If you encounter other forecasters still down-playing this potential by the weekend, I suspect they may not be accounting for how the computer models tend to under-estimate resistance of low level cold air when analyzing over-running/cold air damming events.
In fact, the heavy rain forecasted for southern California later this week is the first wave of this moisture. You can already see this wave of subtropical moisture approaching the west coast on this GOES water vapor loop. Having reviewed the Feb 07 and 08 ice storms, I strongly suspect this energy and succeeding waves will traverse the country in at least 2 batches, the first on Monday 1/26 and the second starting by Wed 1/28. While most areas should see a few inches of snow at the onset, let me emphasize this is not looking like a heavy snow event for Maryland, but primarily sleet/freezing rain.If my analysis pans out, this indicates suggest a long duration of disruptive weather for the Mid-Atlantic certain to impact school schedules, important teacher inservice events, and reporting of 2nd quarter grades. If there is something really important you'd like to finish before the month is out... this is the week to do it.
No comments:
Post a Comment