May 8-21, 2005 Pattern Discussion

[donsutherland1]

Last week’s discussion raised a number of issues of what one might expect for the May 1-14, 2005 period.

• Northeast/Mid-Atlantic: A fresh unseasonably cold air mass could move into the region on Sunday night. Before then, the region could experience severe thunderstorms on April 30. The potential exists for low temperatures to fall to 43° or below in Boston, 45° or below in NYC, and 47° or below in Washington, DC on one or more days next week. Slow warming will likely commence on May 6.

Temperature Anomalies (May 1-5):
Boston: 2.8° below normal
New York City: 5.0° below normal
Washington, DC: 7.3° below normal

Low Temperatures from above criteria:
Boston: 42° 5/5; 43° 5/6
New York City: 44° 5/2; 41° 5/3; 44° 5/4
Washington, DC: 45° 5/2; 38° 5/3; 45° 5/5

• Chicago and Detroit will likely experience a fresh surge of unseasonably cold air for the May 1-4 period. It is possible that both cities could see at least one minimum temperature that is below 40°. Noticeable warming will likely commence on May 5. The May 6-10 period could see high temperatures rebound into the 60s and perhaps 70s.

Chicago saw the temperature fall into the 30s each morning during the May 1-5 period. Detroit saw low temperatures in the 30s each day during the May 1-4 period.

May 1-4 Temperature Anomalies:
Chicago: 10.5° below normal; Trace of snow on 5/2
Detroit: 11.5° below normal; Trace of snow on 5/2; 0.1” snow on 5/3

Warming commenced on 5/5. Chicago reached 70° on May 5 and 75° on May 6. Detroit reached 68° on May 6.

• In the Central Plains, generally cool conditions are likely to prevail through May 3. Afterward, a significant warmup is likely to commence. By May 6, Kansas City and Omaha could see temperatures in the 70s.

May 1-3 Temperature Anomalies:
Kansas City: 13.0° below normal
Omaha: 12.7° below normal

Kansas City saw a warming trend begin on May 4. It began a day later in Omaha. High temperatures in both cities returned to the 70s on May 4 and reached or exceeded 80° on May 6.

• May 1-7 will likely see warmth return to Seattle. May 5 could experience some showers. May 3-10 will likely see temperatures average above normal.

May 1-5 saw the temperature in Seattle average 5.2° above normal and May 6 saw more above normal readings as of mid-afternoon. Seattle received 0.04” rain on 5/2 and 0.02” on May 4.

• May 1-2 will likely see somewhat below normal readings persist in Los Angeles. Afterward, the May 3-5 period should experience near normal or somewhat above normal readings.

May 1-2 saw the temperature average 0.5° below normal at Los Angeles International Airport and 0.5° above normal at Downtown Los Angeles. May 3-5 saw the temperature average slightly above normal at both sites. Total rainfall through May 5 came to 0.01”.

The May 8-21, 2005 Ideas:

Through the medium-term, the NAO is likely to trend back to negative levels prior to mid-month before possibly rising several days afterward. The PNA is likely to average mainly positive through the duration of the period.

• Northeast/Mid-Atlantic: Following the weekend nor'easter, May 8 will likely see temperatures average below normal in New York City and Boston with Washington, DC averaging near or above normal. The May 9-12 period should see Washington, DC through Boston average warmer than normal. One or more days should reach 70° or above in New York City and perhaps Boston. Washington, DC should see one or more days reach 80° and it is even possible that Philadelphia sees one or more 80° readings. Showers could precede cooler air for the May 13-15 period. Warming could commence early the following week.

• The May 8-15 period will feature generally above normal readings for Chicago and Detroit. At least one day could see the mercury reach or exceed 80° in both cities, especially during the second half of this period. Afterward, it could turn briefly cooler before a new warming trend begins by the middle of the following week.

• The Central Plains will likely see above normal readings for the May 8-13 period. Both Kansas City and Omaha should see at least one 80° reading and it is possible that Kansas City’s highest temperature could reach or exceed 85°. After a cooler weekend, temperatures should again be trending higher.

• The May 8-11 period should feature near normal to somewhat cooler than normal readings along with frequent precipitation in Seattle. Total precipitation could reach 0.35” or more. May 13-15 should see warmer readings. The May 16-21 period looks to feature somewhat above normal temperatures.

• Los Angeles should see near normal to somewhat above normal readings during the May 8-10 period. May 11-14 should be warmer than normal.

Looking Ahead for the Pacific Northwest: Is the GFS Ensemble Mean Reasonable?

If one examines the GFS ensemble mean’s forecast concerning 850 mb temperature anomalies for the May 16-20 period, one finds that the idea favors the period as a whole seeing temperatures average near normal to somewhat above normal for the Pacific Northwest. Is this reasonable?

One way to test the reasonableness of the forecast is to examine the PNA. Most of the members of the GFS ensembles forecast the PNA to average in the +0.00 to +1.00 range during this timeframe.

A look back at the PNA since regular recordkeeping began in 1950 shows that there were 23 instances in which the PNA was in this range for two or more days during the May 16-20 period. Temperature anomalies were as follows:

• Below normal or Below normal to near normal: 12 (52%)
• Near normal to above normal or above normal: 11 (48%)

Once ENSO conditions were considered, there were 7 years that fell within the top 15 matches with regard to ENSO region temperatures. The breakdown was as follows:

• Below normal or Below normal to near normal: 2 (29%)
• Near normal to above normal or above normal: 5 (71%)

From that sample of 7 years, there were 4 years where the PDO was positive in April (with the idea of a lagged response between the PDO) or May. The following occurred:

• Below normal or Below normal to near normal: 1 (25%)
• Near normal to above normal or above normal: 3 (75%)

Consequently, historic data appears to support the idea expressed on the GFS ensemble mean that the Pacific Northwest will likely see temperatures that average near normal to above normal during the May 16-20 period.

[TT-SEA]

I am not so sure about a warm period in the Pacific Northwest from May 16-20.

I think the extremely variable MJO may change what would normally be expected.

Here is the 18Z run of the GFS at 300 hours (5/19)...

[img]http://www.nco.ncep.noaa.gov/pmb/nwprod/analysis/namer/gfs/18/images/gfs_500_300m.gif [/img]

[donsutherland1]

TT-SEA,

I'm thinking that the average for the period will be somewhat warmer than normal. At the extended range, the GFS ensemble mean is usually better than the operational model. The MJO is a big wildcard, as it would favor a transition toward cooler weather but perhaps the cool anomalies won't occur until near the end of the period allowing the 5-day average anomaly to be somewhat on the warm side. Time will tell.

[TT-SEA]

Yes... the timing will be interesting.

Still... I think a wet period is coming at some point in the next two weeks. It could be very wet if we get a zonal flow.

[donsutherland1]

TT-SEA,

On that, I don't disagree. After the dry winter, I'm sure the Pac NW won't be complaining about some wet weather.

[donsutherland1]

One final caveat: if the PNA is negative, the outlook would be quite different. Some GFS ensemble members took the PNA negative, so that's certainly a possibility.

[TT-SEA]

Its looking quite likely now that May 16-20 will feature below normal temperatures and above normal precipitation in Seattle.

[donsutherland1]

TT-SEA,

Yes, the models are currently pointing to a cooler period. In the end, the idea will be verified whether it is right or wrong.

[donsutherland1]

On May 6, I noted:

If one examines the GFS ensemble mean’s forecast concerning 850 mb temperature anomalies for the May 16-20 period, one finds that the idea favors the period as a whole seeing temperatures average near normal to somewhat above normal for the Pacific Northwest. Is this reasonable?

One way to test the reasonableness of the forecast is to examine the PNA. Most of the members of the GFS ensembles forecast the PNA to average in the +0.00 to +1.00 range during this timeframe.

A look back at the PNA since regular recordkeeping began in 1950 shows that there were 23 instances in which the PNA was in this range for two or more days during the May 16-20 period. Temperature anomalies were as follows:

• Below normal or Below normal to near normal: 12 (52%)
• Near normal to above normal or above normal: 11 (48%)

Once ENSO conditions were considered, there were 7 years that fell within the top 15 matches with regard to ENSO region temperatures. The breakdown was as follows:

• Below normal or Below normal to near normal: 2 (29%)
• Near normal to above normal or above normal: 5 (71%)

From that sample of 7 years, there were 4 years where the PDO was positive in April (with the idea of a lagged response between the PDO) or May. The following occurred:

• Below normal or Below normal to near normal: 1 (25%)
• Near normal to above normal or above normal: 3 (75%)

Consequently, historic data appears to support the idea expressed on the GFS ensemble mean that the Pacific Northwest will likely see temperatures that average near normal to above normal during the May 16-20 period.

In the following weekly forecast, I realized that the warmer side of this idea was not going to work out. Overall, during the May 16-20 period, Seattle's temperature averaged 1.0° below normal:

May 16: Normal
May 17: 3° below normal
May 18: 2° above normal
May 19: 1° below normal
May 20: 3° below normal

In short, the first idea concerning this timeframe proved incorrect.