INSTRUCTOR PLT OF A PIPER PA28-180 BECAME CAUGHT IN MARGINAL VFR WX CONDITIONS WHEN RETURNING FROM A LCL DEMONSTRATION FLT TO A PROSPECTIVE STUDENT.

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I TOOK A FRIEND (AND PROSPECTIVE STUDENT) UP FOR A DEMONSTRATION FLT. THROUGHOUT THE DAY; I UPDATED MY WX INFO THROUGH DUATS; ATIS; AND FSS RECORDED BRIEFINGS. CONDITIONS SEEMED TO MATCH THE FORECAST: VISIBILITY +6 MI; 2500 FT SCATTERED TO BROKEN; 4000 FT BROKEN OVCST; LIGHT SURFACE WINDS FROM THE S. TEMPORARILY 500 FT SCATTERED. I DECIDED TO STAY LCL. I DID NOT GET A STANDARD BRIEFING; BUT RELIED ON DUATS/FSS RECORDED BRIEFS. TKOF/DEP WAS UNEVENTFUL. FLEW OUT TO THE NE TOWARDS ARLINGTON MUNICIPAL ARPT (AWO); LOWEST CEILING SEEMED TO BE ABOUT 2500 FT BROKEN. I CRUISED ABOUT 1500- 2000 FT AGL. VISIBILITY WAS +6 MI. ENRTE BACK TO THE ARPT (NEAR MARYSVILLE; WA) I TUNED UP ATIS. VISIBILITY 20 MI; 2500 FT SCATTERED CEILING 4000 FT BROKEN(?). I WAS ABOUT 1500 FT AGL. I RPTED IN TO TWR ABOUT 7 MI NE (NORTH EVERETT). A LOW STRATUS LAYER WAS BEGINNING TO FORM; AND AN AREA OF RAIN WAS N AND E OF THE ARPT (BTWN US AND THE ARPT). TWR TOLD ME TO RPT STRAIGHT IN OVER THE BOEING PLANT (1-2 MI N OF RWY 16L/34R). AS I CONTINUED SWBOUND FROM EVERETT; IT BEGAN TO RAIN. VISIBILITY DECREASED TO ABOUT 3 MI. I KEPT VISUAL CONTACT WITH THE GND; FLYING OUT OVER THE WATER (GARDNER BAY; W OF EVERETT) AT 900 FT. I STILL HAD THE BOEING PLANT IN SIGHT. I FOLLOWED THE SHORELINE UNTIL I WAS ON AN EXTENDED FINAL ABOUT 2-3 MI N OF RWY 16L. I CONTINUED IN FOR A NORMAL LNDG. ABOUT 6- 10 MINS AFTER LNDG; THE BEACON WAS ON AND PAE WAS RPTING VISIBILITY 5 MI; CEILING 500 FT BROKEN. I CALLED SEA FSS ON A CELL PHONE TO VERIFY THAT THEY HAD PAE'S WX RPT AND THAT LCL WX WAS MARGINAL VMC AND NOW IMC. WHAT DID I LEARN FROM THIS? SEVERAL THINGS. WE MADE IT BACK OK AND THE SUCCESSFUL OUTCOME OF THE FLT WAS NEVER IN DOUBT. HOWEVER; AS A CFI; I SET A CRUMMY EXAMPLE FOR MY PROSPECTIVE STUDENT! FIRST; I SHOULD HAVE GOTTEN A STANDARD BRIEFING FROM SEA FSS. HOWEVER; THE IMC HAD BEEN UNFORECAST SO I'M NOT SURE HOW MUCH IT REALLY WOULD HAVE HELPED. I SHOULD HAVE PAID MORE ATTN TO THE TEMPERATURE/DEWPOINT SPREAD! I BELIEVE THE TEMPERATURE WAS JUST 1 DEG F WARMER THAN THE DEWPOINT. EVEN AS I CLBED INTO THE AIRPLANE BEFORE TKOF; I FELT THE AIR GETTING A LITTLE COOLER. I SHOULD HAVE NOTICED. I SHOULD HAVE LISTENED TO THAT 'CHICKEN LITTLE' VOICE THAT SAID 'MAYBE WE SHOULD JUST STAY IN THE PATTERN TODAY.' BEING 'CHICKEN' IS A GOOD THING -- IT KEEPS YOU OUT OF TROUBLE. GET HOMEITIS CAN HIT ANYBODY! I NEVER THOUGHT IT WOULD BE ME! I COULD HAVE LANDED AT ARLINGTON MUNICIPAL ARPT (AWO) INSTEAD OF PUSHING ON THROUGH MARGINAL VMC TO PAE. MY WIFE COULD HAVE DRIVEN TO AWO TO PICK US UP. I COULD HAVE FLOWN THE AIRPLANE BACK TO PAE THE NEXT AFTERNOON. FINALLY; IT WOULD HAVE BEEN NICE TO HAVE HEARD AN AMENDED FORECAST AT LEAST MENTIONING THE EXISTENCE OF LOCALIZED IMC. DESPITE MY PIREP AND PAE'S WX RPT; THE FORECAST WAS NEVER UPDATED. EVERY LITTLE PIECE OF INFO HELPS!