January 9-January 22, 2005 Pattern Discussion

[donsutherland1]

Anthony,

Interestingly enough, Seattle was receiving some light snow this morning. As for the magnitude of the cold later this week, my thinking remains the same as earlier that Wednesday-Friday (1/12-14) could be the coldest stretch for such cities as Seattle. Then, lows could fall into the middle 20s. Teens could still occur, but that's not yet certain.

[AnthonyC]

Parts of Western Washington woke up to 2-4 inches of snow...a solid amount for locations at sea level. Local meteorologists say another shot of snow Tuesday night/Wednesday morning. Tuesday-Thursday shows high temperatures in the mid-upper 30s, low temperatures upper teens-lower 20s. Does this verify with latest model forecasts? After that, seems a dirty ridge will develop off California and the PNA goes back to neutral, if not slightly positive.

Anthony

[donsutherland1]

Brief update...

For the most part, the pattern discussion continues to look good. Global indices are trending as described. The modeling is now showing good support for a pattern change that would bring colder than normal readings to the East after January 14.

Some additional points:

With regard to the Pacific Northwest, it is still possible that ahead of one additional shot of cold air, there could be some snowfall (generally around 1/11-13). Afterward, the region could see its coldest readings to date. For Seattle, I believe the lowest temperature will come out around 18° +/- 2°. After the 14th, a warmup should take place as the PNA transitions toward positive.

With regard to the height of the warmth ahead of the passage of a strong cold front in the East, it increasingly appears that the 60° isotherm might make it not just to NYC as I had stated earlier but perhaps to Boston, as well.

Any precipitation along the front should be mainly rain. However, it is possible that in some places, the rain would end as a brief period of snow that could coat the ground. A significant snowfall does not appear likely through the next weekend. Many areas in the East might see no snow at all from the frontal passage.

[donsutherland1]

Anthony,

That seems pretty reasonable given the latest guidance.

[andrewr]

How long do you predict the pattern to stay warm in the west and cold in the east?

[roebear]

Don, have to make a business trip to Missouri for five days, I have a part time snow plow business here near Hershey, PA. So I want to pick the week I have the least snow in my area to go. I have two choices for the weeks I have to go, either Jan 24-28 or February 21-25. Now I know it is next to impossible to predict weather for five day period as far out as February 21-25. My inclination is the five days in January 24-28 have a good chance of snow in this area, so tend toward the farther out February 21-25 time period.

I am not expecting you to predict so far ahead either, but was wondering if you have any statistical figures for analog years with similar weather patterns. If you do, which week has the least statistical probability for snow in Southern PA?? We have hardly had a flake so far, so probably a blizzard warning for any week I pick to be out of area,

[donsutherland1]

Andrewr,

These kinds of pattern changes often last 2-3 weeks. Per a number of the situations that were similar to the ongoing evolution (based on ENSO regional profiles), the warmth generally persisted through mid-February. After that, one saw more normal conditions occur (but normal temperatures had risen by then). There were a few occasions where February 1-15 saw cool conditions return then the trough shift to the East again after mid-February.

This is based on analogs. Right now, the timeframe is beyond the horizon of many of the models. FWIW, per the RSM and GSM, the pattern change, once it occurs, is expected to last at least through the January 29-February 5 period (which is as far as these models go).

[donsutherland1]

Roebear,

By what date do you need this info. I'll need to do a little research into the analogs for a location at least reasonably close to Hershey. Hopefully, I can have this info. by no later than Tuesday.

[AnthonyC]

Let's think about this...

The pattern shifted from the east to the west during Xmas. So we've been in this pattern for almost three weeks. Once the pattern shifts back to the east next weekend, you probably can expect two-three weeks of warmer/drier weather in the west, colder/winter weather in the east. But I wouldn't be surprised by the beginning of February if the pattern returned to the west. There's also the possibility we could go into a zonal flow...that's usually a boring weather pattern, but does bring alot of precip across the country.

Anthony

[andrewr]

Anthony, mostly I'm just thinking of our mountains. If it is 50+ degrees and sunny everyday for 2 weeks then there wouldn't be an inch left in most areas. I'm doubting that will happen, but I'm sure the mountains are going to continue to suffer.

[roebear]

Tuesday will be fine, thanks a bunch for whatever you can dig up!!

[KWT]

just thought i'd say NAO is set to go negative,maybe very negative soon:

[donsutherland1]

Roebear,

After examining the global indices against past seasons with similar readings, I believe the greater threat of plowable snow this season could well occur during the February 24-28 period.

To be sure, the average snowfall and average number of events is more favorable in the January 21-31 period. However, the averages are skewed on account of two blockbuster January 21-31 periods:
1987: 16.3"
1966: 24.3"

Another thing to consider is the MJO possibly switching to a wet phase around mid-February. This could also translate into a heightened risk of snow as the month approaches its end.

In order to compensate for slight timing differences between possible analogs (they are used for guidance not specific point forecasts), I examined the January 21-31 and February 18-28 periods.

I used 3 samples for Harrisburgh, PA:

• Test 1: Seasons similar to the current one with regard to the December ENSO regional anomalies: 15 seasons
• Test 2: Seasons similar to the current one with regard to both the December and October-December ENSO regional anomalies: 10 seasons
• Test 3: Seasons similar to the current one with regard to the December and October-December ENSO regional anomalies that also saw a pattern change to colder for the 2nd half of January: 7 seasons

The data revealed the following:

Test 1:
January 21-31: Average snowfall: 4.2"; 23 events (15 seasons); Seasons with 3" or more snowfall: 5/15 (33%)
February 18-28: Average snowfall: 3.4"; 20 events (15 seasons); Seasons with 3" or more snowfall: 7 (47%)

Advantage to January 21-31: 23.5% more snowfall; 15% more events
Advantage to February 18-28: 40% more seasons with 3" or more snowfall

Test 2:
January 21-31: Average snowfall: 5.6"; 16 events (10 seasons); Seasons with 3" or more snowfall: 3/10 (30%)
February 18-28: Average snowfall: 4.2"; 13 events (10 seasons); Seasons with 3" or more snowfall: 6/10 (60%)

Advantage to January 21-31: 33.3% more snowfall; 23.1% more events
Advantage to February 18-28: 100% more seasons with 3" or more snowfall

Test 3:
January 21-31: Average snowfall: 7.3"; 14 events (7 seasons); 3/7 (42%) seasons with 3" or more snowfall
February 18-28: Average snowfall: 4.6"; 9 events (7 seasons); 5/7 (71%) seasons with 3" or more snowfall

Advantage to January 21-31: 58.7% more snowfall; 55.6% more events
Advantage to February 18-28: 67% more seasons with 3" or more snowfall

Note: An event is constitutes any period where 0.1" or more snow fell.

Finally, it should be noted that analogs are far from precise predictors. They do offer some guidance and it appears that they reasonably support the February 24-28 period as having the better opportunity for a plowable snow.

[donsutherland1]

Brief update...

On December 31, 2004, I had <b>noted</b>:

<i>My quick thoughts on the NAO, PNA, and AO:

• AO: Begins a steady decline after January 7 and goes negative sometime between January 10-15.
• NAO: Begins a gradual decline but holds strongly positive through January 7. After January 10, it heads toward neutral territory.
• PNA: A slow rise occurs but it holds mainly negative through January 10 and then perhaps goes neutral a few days later. </i>

These assumptions were key to the idea that a mid-month pattern change would evolve as highlighted by analogs and discussed in greater detail at the beginning of this thread.

With regard to these key teleconnection indices, things have trended in a favorable direction:

∙ The Arctic Oscillation has just gone negative (well within the timeframe noted).
∙ The NAO is now arguably neutral at less than +0.5 and falling.
∙ The PNA remains strongly negative (< -2.0) but is now rising.

The trends in the GFS ensembles (in my view, the growing support among ensemble members is of greater importance than the ensemble mean values for the indices), remain favorable. Now, the NAO and AO could go negative, briefly bounce positive and then go negative for a longer period of time. I'm reasonably confident per analogs that the longer duration negative event will occur. The PNA could also be sustained positive though it could be dented from time to time by strong systems moving toward the Pacific Northwest. The 10-day Canadian ensemble also implies a + PNA.

As noted earlier in this thread, "Of the 11 seasons in which the average ENSO regional temperature was less than +0.5°C from that of December 2004 (Regions 1+2, 3, 4, and 3.4), 7 (64%) saw a trough predominate in the East for the January 16-31 period. Moreover, for the regional profiles most similar not just to December but also over the past 3 months, 7 of those 10 (70%) seasons saw a trough predominate in the East during the January 16-31 period. The two seasons that saw similar December and October-December ENSO setups and January 1-6 500 mb anomalies also saw a trough predominate in the East during the January 16-31 period."

I believe the changes one is now seeing in the global indices are the first milestones indicating that one is traveling down this meteorological road and that the analogs have helped lend some clarity to a very uncertain time with regard to the evolving pattern. Now, the endgame seems to be breaking in a fashion as had been hinted at by the analogs. Thus, at this point in time, I remain comfortable with the idea that "A pattern change is nearing and it should ensure that the second half of the month sees a predominant trough in the East/ridge in the West configuration."

In the updated pattern discussion (January 16-31), I'll take a closer look at how things are evolving and incorporate the January 5 ENSO regional data into the picture. If the January 12 data becomes available by that time (either tomorrow or Saturday), I'll employ it, as well.

For now, those in the East who have grown tired of the unseasonable warmth--Washington, DC hit a springlike 70° earlier this afternoon--and lack of opportunity for snowfall, things are looking better. However, for spring weather lovers, a fall from the balmy January Paradise may be near at hand.