Short Term - Week of 12/6:
There is not much going on this week. In last week's Mo Mo, I mentioned there is a possibility of a Nor'easter to effect our area Tuesday-Wednesday this week. The pattern does not support one.
18z GFS 500hPa Vorticity valid 1am Tuesday:
500mb vorticity is a useful tool that measures the amount of "spin" in the atmosphere. I often refer to it as upper level "energy" or "short waves" that (if positive advection) can lead to surface low development (surface low = an actual storm). Notice the potent level of vorticity I circled over our area Tuesday morning. The reason why there is not a deep low pressure system sitting off our coast at this time is because the overall flow is not amplified, meaning you see no trough's or ridge's on a 500mb map. The very weak / disorganized trough along with the fast flow is the main reason why this Nor'easter will not happen. Instead, we're likely to see clouds with light rain showers.
Expect temperatures to be in the low to mid 50's the entire week other then Tuesday. In fact, they'll be even higher come the weekend.
18z GFS 500mb Vorticity valid 1am Sunday the 13th
A trough axis with very strong vorticity embedded is expected to be located over Texas this weekend. A surface low pressure system will form and track somewhere between Indiana and Western PA. The end result for our area will be much above normal temperature departures thanks to the High Pressure sitting off the east coast helping to pump up mild Gulf air from the Southeast.
12z EURO temp anomalies valid 1pm Saturday
The process I explained above actually takes place sooner than Sunday. The troughing will take shape over the western U.S. beginning Friday. This means positive heights are expected to take over the central and eastern U.S. Friday through the weekend. The EURO is advertising temperature departures of 20* to as high as 30* above normal from TX to eastern Canada. I would not be surprised to see NYC reach 60 degrees both Saturday and Sunday this weekend. Meanwhile, the western U.S. continues to see cold temps and frequent snowstorms.
Medium Range - Week of 12/13:
12z EURO 500mb height anomaly valid 7am Sunday 12/13
This graphic is intended to show another perspective. The 1st system over Texas leading to the ridging over the eastern U.S. is the one I just finished talking about. By Monday the 14th, the cold front associated with this storm will reach our area (it's possible the timeline gets moved up to Sunday) to bring heavy rain and possibly high winds. I do not buy into the high wind forecast yet but I do think we'll see decent rain next Monday.
The 2nd system entering the Pacific Northwest should bring more rain to the area Thursday-Friday (17th-18th) next week. I will speak more about this system in next week's Mo Mo.
Long Term - Initial Thought's on Christmas:
12z EURO Ensembles 500mb height anomaly valid 7am Wednesday 12/16
Keep in mind when looking at these images model verification's in the long range are not great. That said, we seem to be in a persistent pattern that does not intend on changing anytime soon. The EPS suggest the western U.S. will continue to get inundated with upper level trough's, or colder than normal temperatures, while the east stays average to slightly above. Part of the problem is the cold moderates as it travels east. The Alaskan Vortex prevents the Pacific ridge from amplifying into the EPO/PNA domains so the flow remains flat and fast.
12z EURO Ensembles 500mb height anomaly valid Saturday December 19th
The EPS show no change as we head deeper into December. This is also in line with what the EURO Weeklies are showing. Over the past weekend, the GFS OP and GEFS seemed to take a step in this direction as well. At one point, the GFS OP was really overdoing the cold weather across the U.S. Given the strong El Nino regime we're under, in conjunction with an unfavorable Stratosphere and Tropical Forcing, this is not surprising to me. I predicted December to be warmer than normal given the historical nature of strong El Nino's in west-based QBO years. It's going to take time for Nino to weaken to moderate levels. Once it does, the Sub-Tropical Jet Stream will not be the dominant force in our overall pattern.
I spent a good amount of time in last week's Mo Mo analyzing the current state of the Stratosphere. I do not want to spend as much time again this week, but do want to point out that the upper Stratospheric warming guidance was showing a couple of days ago does not look as impressive in the recent runs.
Here is what the OP GFS showed this past Saturday afternoon at 10hPa:
Now here was Sunday afternoon's run:
Noteworthy:
1. The magnitude of the ridging or warmth over Siberia is not as strong
2. As a result, the core of the PV is not as elongated / displaced
Temperatures at 50hPa (mid-Stratosphere) are running colder than normal (normal = solid black line) and are forecasted to get even colder this week. This indicates the PV remains strong and well organized. It will take a couple of potent warming events to weaken it to a point where it has an effect on our weather at the Troposphere.
It's also worth mentioning the MJO is forecasted to go into phases 3-4. The convection we saw these last couple of days over the Dateline is expected to propagate west toward the western Tropical Pacific while the Indian Ocean continues to see frequent thunderstorm activity. Historically, an MJO in phase 3 during El Nino years result in the following 500mb map:
It's not a perfect match to what current guidance is showing in the long range. The general idea is correct with positive heights over central / eastern U.S. and Canada. while also showing the western U.S. trough. Overall, MJO influence in strong El Nino years is not as strong as ENSO neutral or La Nina regimes. It's not one of the signals I'll be paying much attention to this winter.
Christmas Forecast:
I will likely update this forecast in the week's to come, but my initial thought's are it will be average to slightly below average Christmas Day. Depending on where the baroclinic zone sets up, I can see how a winter storm effects our area. However, in order for the baroclinic zone to be south & east of us, we'll need to see either a -EPO or +PNA begin to show up on the models. The odds of one of these features developing in this locked in pattern is pretty slim. The reason why I am leaning below normal is because the two low pressure systems expected to cut next week could create "traffic," or confluence, north of us that prevents a tertiary low from cutting. Instead, a weak trough could settle in over our area. By next week we'll be able to see what some of the long range models are showing. Right now, none of the major global models are in range.
Thanks for reading. Have a great week!
Best,
Francesco Paparatto
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